Philip Hammond, shadow Treasury chief secretary, said the Tories backed the idea "in principle" but it had to be seen in the context of the UK's huge fiscal problems.

But Mr Hammond denied that his party had already made up its mind not to give Holyrood borrowing powers, as recommended by the Calman Commission on devolution.

The Tories were "open minded" on the issue and were still studying it, said Mr Hammond.

He said: "We are looking at the commission's proposals.

"These are very difficult times, and it's not clear that right now is necessarily the best time to implement that specific proposal.

"But we are certainly in favour of the broad thrust of the Calman Commission's recommendations."

He went on: "It might not be something that happens in the next year or so.

"It's a proposal which has merit in itself, but we are in very extraordinary times at the moment and there are overarching macroeconomic considerations that have to be taken into account."

He also said his party's plans for a public sector one-year pay freeze would not affect the devolved Scottish Government directly - but would do so indirectly, by affecting the money allocated to Scotland through the Barnett Formula.

"In the case of Scotland, the English settlement will flow through, via the Barnett Formula, into the block grant," he said.

"It will be for the Scottish Government to decide how it uses its money, and if it chooses not to implement the pay freeze and can find the money somewhere else, that is its entitlement under the devolution settlement.

"But the budgets for England will be set on the assumption of a pay freeze in 2011."