The Conservative leadership is in private talks with international credit- rating agencies to persuade them that it is “deadly serious” about dealing with Britain’s debt mountain, George Osborne has disclosed in an interview with The Times.

The Shadow Chancellor said that preserving the country’s AAA rating was “incredibly important for our international reputation and for the cost of our debt”. He added: “We must do what we can — if we are elected — to preserve that rating. I think there will be a lot of pressure on a government early on to be able to demonstrate they have a credible fiscal plan.”

He said that the Government was holding similar talks.

Mr Osborne said that a Budget would be held within weeks of an election victory to “reassure domestic business and international investors that this country can pay its way”.

His disclosure of the talks sheds new light on the Conservative political strategy to confront Britain with its austere future. He rejected criticism that raising doubts about the nation’s creditworthiness in public was irresponsible. “Look, this is the real world. And the real world is that these things are being discussed,” he said.

He added that it was a “statement of facts” that three agencies, Moody’s, Fitch and Standard & Poor’s, had all voiced concerns about Britain’s ability to pay off its record debts. Mr Osborne refused to say with whom he had held talks, but added: “They are private conversations, but they want to know what our thinking is. They want to hear that we are serious about this.”

In an interview the day after he set out the first cuts of a Tory government, including a pay freeze for millions of workers, Mr Osborne revealed:

- He is holding the threat of a windfall tax if banks refuse to restrain excessive pay and bonuses.

- He will not pledge to reverse the rise in national insurance of 0.5 per cent, due in April 2011, at the next election.

- He will spell out the timetable for a “more ambitious” and faster plan to halve the deficit after next year’s Budget, but before polling day.

- He is already recruiting figures to staff a new watchdog to police spending restraint across government.

- He will cut “tens of billions of pounds” in failed, inefficient and wasteful spending.

Mr Osborne admitted that the package he set out on Tuesday, which he claimed would trim £23 billion from the deficit, was “not the whole solution” and that a Conservative government would slash “tens of billions” from Whitehall budgets after the election.

He disclosed that he has been talking to the last Conservative Shadow Chancellor to arrive at Number 11, Lord Howe of Aberavon. Asked whether he would imitate Lord Howe and bring in a windfall tax on the banks, he said: “I am absolutely clear that the money provided by the taxpayer should not be distributed in excessive profits and excessive pay and bonuses. We reserve the right to take action through the tax system. The credit crunch may be coming to the end in the City, it’s not coming to an end in places like Manchester. The banking system is not functioning normally.”

Asked whether he was worried that his announcements on pension-age rises and a public sector pay freeze risked turning public opinion decisively against the Tories, he said: “David Cameron and the Conservative Party are at their best when they are making the argument and setting the terms of trade in British politics.”

He rejected those in his party who worried that he could not remain both Shadow Chancellor and oversee the Tory election campaign. “I don’t see preparing for the Budget as somehow entirely separate from persuading the public to put us into office, because they are closely linked.”

Meanwhile, Philip Hammond, the Shadow Treasury Chief Secretary, said that Britain could suffer “a couple of years of significant pain” before the economy returned to good health.

He said that the Conservatives would axe more than 20,000 jobs in Whitehall and quasi-public bodies.

He told the Financial Times: “Nobody likes to be told that there’s going to be a couple of years of significant pain for everybody, but we are being honest with people and the prize is great.”

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