Conservative Ian Lang - now Lord Lang of Monkton - said that Scotland had been on a "slippery slope" to independence since the inception of the Scottish Parliament in 1997.

He claimed devolution had impacted negatively on the UK because the latter had become fragmented, and people felt Scottish or English rather than British.

Lord Lang said he believed the divisions would become more marked if and when the UK encountered economic difficulties.

People no longer see themselves as British, they think of themselves first as Scottish or English and the idea of being British is gradually disappearing

Lord Lang

Speaking on STV's Seven Days programme he said: "I feel that the problems are not the issues of the ability of the MSPs, or the time that they spend debating their own holidays and pay, and their political agenda - obviously one can quibble with those things.

"My concern is more with the underlying structural position.

"I don't believe you can have a devolved parliament of this kind within a unitary sovereign state, other than on a federal basis, and nobody has been able to contemplate a federal United Kingdom."

Despite this he said that he wanted the Scottish Parliament to succeed.

However, he also feared how future economic slumps would impact on Holyrood's relationship with Westminster.

Political potential

"People no longer see themselves as British, they think of themselves first as Scottish or English and the idea of being British is gradually disappearing," he said.

"Now that may not seem to matter very much, but when the crunch comes, when issues get difficult, when times are hard, my feeling is that is when it will become apparent just how much the United Kingdom has fragmented."

Earlier this month the current Scottish Secretary, Helen Liddell, said devolution had strengthened Scotland's standing in Europe.

She said that the country had gained economic benefits from the devolution settlement.

John Adams: "Parliament has been a success"

A book marking the fifth anniversary of the Scotland Act - which created the Scottish Parliament - will be published on Monday.

Published by the Institute for Public Policy Research and Economic and Social Research Council, it is expected to argue that parliament is being held back.

John Adams, the report's author, told BBC Scotland that Whitehall did not pay enough regard to the work carried out by the devolved institutions.

He said: "The parliament has been a success but the next challenge is to fulfil its potential. If there is any area which has failed to grasp the importance of devolution it is Whitehall.

"People who work in Whitehall sometimes don't pay sufficient regard to the, shall we say from their perspective, more peripheral parts of the United Kingdom.

"I'm afraid that they don't fully understand that things now can be done and are being done differently in Scotland and Wales.

"They just have to factor that into their deliberations and their strategies and their thinking a little bit more effectively that they are."