He also told peers it was a "matter of great regret" that Scots living in other parts of the UK would not be given a vote in September's referendum.
Lang, the former Conservative Scottish Secretary, had been urged to consider withdrawing an "ill-judged and offensive remark" about Britain's war dead in a planned speech.
The peer questioned whether the proposed break-up of the Union between Scotland and England by the SNP would dishonour the sacrifices made by soldiers.
Tory HQ last night put out in advance remarks expected during a House of Lords debate on the referendum.
In them, Lord Lang plans to say that for generations Scots and English have lived alongside each other, sharing a British heritage. "They have fought shoulder to shoulder in the battles of the past three centuries and they still serve together today."
He added: "Together, they built and administered the Empire before turning it into the Commonwealth with Scots very much to the fore. Must they now - both Scotland and England - disavow that shared history?
"And does not that dishonour the sacrifices, made in common cause, of those who died for the United Kingdom, a nation now to be cut in two if the present generation of Scottish Nationalists have their way? There is nothing positive about a campaign that would destroy so much."
Keith Brown, the Scottish Government's Veterans Minister and an ex-Royal Marine who served in the Falklands War said: "This is a very ill-judged and offensive remark by Lord Lang, who used to argue against having a Scottish Parliament in equally lurid terms; which contributed to him being defeated by the SNP.
"People across these islands and throughout the Commonwealth made the ultimate sacrifice in common cause, which is respected and honoured by all and always will be."
He added: "It is entirely a matter for him but Lord Lang may wish to reconsider his speech and withdraw the section which will cause offence and hurt to many; and apart from anything else actually damages the No campaign."
A Conservative spokesman made clear Lord Lang stood by his comments but insisted Central Office would not get into a running commentary on the matter.
Lang also warned that Britain's international prestige and influence would "crumble" if Scotland were to vote for independence in a referendum later this year.
He said the possible break-up risked becoming like an "increasingly hostile divorce".
And he suggested that the destructive influence of the vote could be used instead to re-invigorate the UK with a positive alternative for Scotland's place in it.
Opening a major debate on Scottish independence with more than 40 speakers, Lord Lang warned that the "trauma of a broken union" would shake all parts of the UK.
"The once-united kingdom would shrink not just physically but in the eyes of the world.
"Others would see it as diminished - diminished in size, diminished in population, diminished in strength and diminished in authority.
"The mother of parliaments would be viewed as unable to hold itself together.
"An historic partnership of peoples would seem to be crumbling and Britain's international prestige and influence would crumble with it.
"Our standing in the Commonwealth would change, our standing in Europe, Nato, the UN, the World Bank, the World Trade Organisation would also change".
Lord Lang said the UK had so far been a "magnificent success story", forming a "highly efficient single market" and asked: "Why put that at risk?"
He said it was a source of great regret that so many expatriate Scots were disenfranchised in the September referendum.
"North and south of the border, within two generations, countless numbers of Britons could become foreigners to their kith and kin."
Both England and Scotland were "woven into the fabric" of the UK. To disavow that shared history would "dishonour the sacrifices made in common cause of those who died for the UK".
He said there was nothing positive about an independence campaign that would "destroy so much" and leave Scotland a "competitor rather than a compatriot".
It would "risk becoming like an increasingly hostile divorce in which the parties continued to live next door to each other afterwards".
Lord Lang pointed to Bank of England governor Mark Carney's warning yesterday that sharing sterling between an independent Scotland and the rest of the UK could lead to eurozone-style crises unless firm foundations were put in place.
The former minister told peers that Scotland would not have a viable central bank, or be able to print money in a crisis and be a lender of last resort.
It status would have changed from that of "partner" to "dependency".
Lord Lang said it was time to put the "politics of grievance behind" and turn the "challenge of separation into the opportunity for reinvigoration" with a "new unionism" that united the UK and brought constitutional stability to it.
Speaking of the break-up of Britain proposed in the referendum, he said: "This destructive, negative and irreversible process does not need to happen.
"There is a positive alternative for Scotland and all of us within the UK."
Labour former Scottish first minister Lord McConnell of Glenscorrodale said devolution had not only been right for Scotland but good for Scotland.
Lord McConnell said he had always believed that a strong devolved parliament which had real legislative powers provided the "best way forward" for Scotland and the UK.
"We need to move from the divisive and rather negative debate that has been taking place in Scotland to a well-informed, high-level debate over the next six months that allows people to make the right choice," he said.
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Liberal Democrat Lord Steel of Aikwood, a former leader of the Liberal party and presiding officer of the Scottish Parliament, said the issue of independence was one of principle rather than pragmatism.
He said: "The debate in the three years in which we have been discussing this matter has tended to focus on the issue of whether Scotland would be better or worse off as an independent country.
"But I would argue that that is not the right question. If it is the right thing to do to separate off from the United Kingdom and become independent then surely the cost of doing it is immaterial.
"The question is, is it the right thing to do and I would argue not."
Labour former defence and Scotland secretary Lord Browne of Ladyton said the UK had a "unique" trust in intelligence matters with the United States.
He said without the intelligence the country would be "much hampered in containing the 21st century threats we face".
"It is essential to our security," he said. "It is improbable that an independent Scotland, particularly one intent on unilateral nuclear disarmament, would enjoy the same relationship."
Tory former chancellor Lord Lamont of Lerwick warned: "Scottish independence would diminish what remains of the UK in the eyes of the world.
"It would be the end of Britain. It's often forgotten that the name Britain only came into existence after the Act of Union and the name makes no sense if the northern part of this island were to be removed."
Lord Lamont said the loss of Scotland would diminish the UK internationally and have an effect on "our standing on international institutions" with fewer votes in the EU.
Turning to the financial implications, he said: "Independence without your own currency is a very constrained form of independence."
If Scotland had its own fiscal system determining its own deficit, it would have an effect on the rest of the UK, the Bank of England and monetary policy, he added.
"If Scotland runs an excessive deficit ... that will have an effect on interest rates for the rest of the UK. So there would have to be some agreed fiscal limit on borrowing by an independent Scotland."
An independent Scotland would have to pay a higher rate for borrowing "simply because it would have no track record and there would be uncertainty about what sort of fiscal policy it would follow".
Lord Lamont warned: "Ripping the blue out of the Union Jack would be a wretched business which would do nobody any good at all."
Labour former Scottish secretary Baroness Liddell of Coatdyke said an independent Scotland would have "porous borders", which would be extremely difficult to secure with implications for the security of the rest of the UK.
"We are talking about separation and we are talking about divorce," she said, adding that it is often the weaker party that comes out of it worst.
Former leader of the Scottish Conservatives and MSP Baroness Goldie said the partnership between England and Scotland was relevant and a success.
"Don't think that the independence referendum is Scotland's business alone," she told peers in her maiden speech.
"The whole of the UK is affected by this debate. Wherever you live in the UK, if you value it, then we all need to step up to the plate to keep it. We are better together and now is the time to stand together."
Labour's Lord Robertson of Port Ellen, a former defence and Scotland secretary and secretary general of Nato, said the campaign for independence was "surreal".
"We are not some oppressed colony," he said. "We are not as Scots discriminated against, we are not disadvantaged inside this union.
"In fact, Scotland is the second most prosperous part of the United Kingdom after the south east of England."
He said Scots played if anything a "disproportionate" role in British life.
"We are not some persecuted minority yearning to escape oppression," he added.
Tory Baroness Neville-Jones, a former security minister and chairman of the Joint Intelligence Committee, warned about threats of cyber attacks and organised crime.
"The whole of the UK benefits from the security umbrella which runs from the centre, but this of course would change and it would change radically in the event of independence," she said.
Tory former sports minister and ex-chairman of the British Olympic Association Lord Moynihan warned about the impact on British sport.
He said the nation owed a "huge debt of gratitude" to Scottish athletes for the contribution they made to Team GB in the London Olympics and other events.
"To break the bonds which connect us would be deeply damaging to the wider interests of British sport ... and not just the athletes of Scotland would be the losers," he said.