SCOTS voters are to be offered the promise of a refreshed Union in next year's independence ­referendum as the start of a wider shake-up of Britain's constitution, a senior Liberal Democrat has claimed.

Lord Purvis of Tweed, a former MSP, has been tasked by his leader Nick Clegg to begin the process of agreeing a cross-party deal to give the Scottish Parliament much greater tax powers as a way of bolstering the anti-independence campaign in the run-up to Referendum Day in September.

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The drive to form a united front, dubbed the "September 19 strategy", is meant to stop Alex Salmond and the Yes camp from framing the referendum debate on their terms, that is, a choice between independence and the status quo but, rather, frame it in terms of "a refreshed Union versus separation".

The peer said there had been high-level engagement between the three pro-UK parties towards creating a unified position.

But he explained Scottish Labour and the Scottish Conservatives still needed time and space to work through their approaches and, once they had done that by next spring, a concerted effort would be made to create a united front on more powers for Holyrood post the referendum.

The thinking is that any agreed document on more powers - which from a LibDem perspective would see a pledge to transfer virtually all tax powers and some welfare powers to Edinburgh - would enable the pro UK parties to say that a No vote was not the end of a process but the start of a new, enhanced phase of devolution.

A No vote would be regarded as tacit approval of the cross-party consensus and pile pressure on Prime Minister David Cameron to make good his suggestion that a rejection of independence would lead to a stronger Scottish Parliament.

Following a No vote, it is suggested there would be a two-stage process: a snap Scottish constitutional convention to fine-tune the details of a stronger Holyrood followed by a UK constitutional convention in the next Westminster Parliament, which would include the Scottish proposals and also cover the so-called English Question, constituency boundaries and Lords reform.

The Lib-Con Coalition Government has agreed during the next two years to enhance Holyrood's financial powers with a new Scottish rate of income tax and borrowing powers worth £5bn.

Scottish Labour has said it is minded to agree to more tax powers for Holyrood while the Scottish Tories are undertaking their own review on how to bolster the Scottish Parliament's financial responsibilities.

While pro-UK leaders see great political advantage in forming a united front on more Holyrood powers in the latter stages of the referendum campaign, they realise it could be a step too far for some colleagues, particularly within Labour and the Tories, who feel the push towards much greater devolution is being driven by the Nationalists' agenda.

Lord Purvis claimed that a united front from the three pro-UK parties would create a strong push towards selling the policy of a "refreshed Union" and he was optimistic that it could be achieved.

"The real prize after the referendum is that this process will secure the long-term relationship between Scotland and the rest of the UK for the rest of our lifetimes. We should not lower our sights from that ambition.

"The goal will be refreshing the United Kingdom and voters will be comfortable with it and see, through the referendum and then the next General Election, that they have endorsed that vision. It will be a uniquely British form of federalism," he added.

Labour peer George Foulkes said the idea of a united front among the three pro-UK parties on enhanced tax powers for Holyrood had "attractions" but needed to be part of a "longer term plan" for constitutional change in Britain.