Douglas Alexander and Jack Straw will tonight use a reception in Whitehall that celebrates the 300th anniversary of the Act of Union to counter Nationalist attacks on the UK.

Douglas Alexander and Jack Straw will tonight use a reception in Whitehall that celebrates the 300th anniversary of the Act of Union to counter Nationalist attacks on the UK and highlight the Union's economic benefits to Scotland.

Before an audience of MPs and peers, the Scottish Secretary and Leader of the House of Commons will unveil a commemorative £2 coin struck by the Royal Mint at a reception in Dover House, headquarters of the Scotland Office, to mark the signing of the act three centuries ago.

George Reid, presiding officer at the Scottish Parliament, will also be present.

The cabinet ministers' defence of the Union follows that of Chancellor Gordon Brown, who at the weekend criticised "an opportunist coalition" of Nationalists and Tories, which was putting the UK at risk.

Mr Alexander will note that of the 25 articles in the 1707 treaty, 15 related to economic matters and that this aspect of the Union is one that endures with the Scottish economy being a fundamental part of a strong UK economy and that nearly two-thirds of exports from Scotland go to the rest of the UK.

He will say: "Today, Scotland is benefiting from the economic stability of the UK, which has seen more Scots in jobs than at any time in our history.

"Our nations have grown closer over the past 300 years with ties which are deepening still.

"Now half of Scots have family living south of the border and almost one in 10 of the people living in Scotland were born in England.

"Our history proves that unity is strength."

Mr Brown, who is also Master of the Royal Mint, had been expected to be at the launch but, due to a trade mission to India, he will not attend.

Last night, his absence was seized on by the SNP, who, The Herald was told, will not be represented at the Dover House event. Pete Wishart, MP for Perth and North Perthshire, received an invitation but, according to his office, he had already left for Africa on party business.

Alex Salmond, party leader, said: "It is highly symbolic in terms of lack of enthusiasm for the Union in Scotland that Brown should decide to hold a party in London to launch a £2 coin and then proceed not to turn up.

"It's another indication that Brown's new-found fondness for the Union is based less on national interest and more on self-interest in an attempt to grab the keys to Number 10."

The Banff and Buchan MP suggested it showed a "certain lack of sensitivity to launch a commemorative coin in London, considering the signing of the Treaty of Union took place in Scotland".

Sir Menzies Campbell, the Liberal Democrat leader, praised the "political and economic stability" brought about by the Union.

However, David Mundell for the Tories urged MPs to sign his parliamentary motion, which praises the Union as "one of the greatest political success stories of modern European history".

He lambasted Mr Brown for doing "virtually nothing" to mark the tercentenary.

"It's no good saying on one day you want to promote the Union and then only being prepared to come up with a £2 coin to mark the event on the next.