A pledge by David Cameron and the Labour and Liberal Democrat leaders that Holyrood will get new powers if voters reject independence has been branded a "desperate bribe" by one of the architects of the Scottish Parliament.

Canon Kenyon Wright, who chaired the Scottish Constitutional Convention which spearheaded the campaign for a devolved assembly in the 1990s, said transferring more power from Edinburgh to London is "not an answer to Scotland's needs".

He was speaking after the leaders of the Conservatives, Labour and Liberal Democrats at Westminster made a formal joint declaration promising to give Holyrood more tax-raising powers and greater responsibility for social security after the 2015 general election.

The three parties have already set out their own proposals for enhancing devolution, and Mr Cameron, Labour leader Ed Miliband and Liberal Democrat leader Nick Clegg have now all signed a joint declaration which states: "We support a strong Scottish Parliament in a strong United Kingdom and we support the further strengthening of the Parliament's powers."

The three go on to promise to "strengthen further the powers of the Scottish Parliament, in particular in the areas of fiscal responsibility and social security".

The declaration continues: "The Scottish Labour Party, the Scottish Conservative and Unionist Party and the Scottish Liberal Democrats have each produced our own visions of the new powers which the Scottish Parliament needs.

"We shall put those visions before the Scottish people at the next general election and all three parties guarantee to start delivering more powers for the Scottish Parliament as swiftly as possible in 2015.

"This commitment will deliver a stronger Scottish Parliament in a stronger United Kingdom."

Mr Clegg said the declaration is a "significant statement of common purpose from the parties that have shaped Scotland's home rule journey".

He added: "From the constitutional convention to the Scotland Act 2012, Liberal Democrats have worked with others and delivered. We are unflinching in our commitment to building a stronger Scotland within the UK, strengthening areas of fiscal responsibility and introducing new powers over social security.

"It is crystal clear that voting No to leaving the United Kingdom is also a positive vote for more powers within the UK family. That's the best of both worlds."

But Canon Wright, a retired Episcopalian clergyman, said: "The latest 'offer' of more powers from the unionist parties looks suspiciously like a rather desperate bribe - but it is not an answer to Scotland's needs.

"The central issue at stake in the referendum is simple: Where should the ultimate power to make decisions that affect the lives of people in Scotland lie - at Westminster or in Scotland?

"Messrs Cameron, Miliband and Clegg cannot be surprised if I now see independence as the only way left open, to give Scotland power over her own affairs that is both complete and secure. Their way offers neither."

He insisted devolution is "incomplete" as it "leaves vital areas directly under the control of Westminster", and added: "Second, devolution is insecure. The wording of today's 'offer' is very revealing. It indicates that greater powers would be 'granted to Scotland'. Granted indeed? By whom?"

He added: "Devolution is power, not as a right, but by gift - or more accurately by loan, since gifts can't usually be taken back. Westminster would retain the permanent power to grant, alter, or rescind. Devolution leaves the UK as a whole fundamentally unreformed."