A not-so-brief Brief Encounter: Beleaguered Prime Minister forced to share a train to Wales with the Tory leader

But it got even worse for the beleaguered Prime Minister when he learned he would have to share a train all the way to Wales with David Cameron.

The Tory leader was posing for photographs while waiting for his train to leave London's Paddington station as Mr Brown swept past with entourage.

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The pair ended up in in adjacent carriages as they headed to South Wales to hit the campaign trail for next week's local elections. Mr Brown was heading for Swansea while Mr Cameron was going to Barry.

When Mr Cameron saw the seats reserved for the Prime Minister, he said: "It would be fun to have a chat."

He spoke after it was revealed the Tories have opened up their biggest lead since Margaret Thatcher's heyday more than 20 years ago, a poll claimed last night.

The 18-point gap is out of step with some recent surveys but, if correct, appears to underline the damage done to Gordon Brown by the 10p tax row.

It added to the Prime Minister's woes as his attempt to buy off rebels threatened to unravel over fears that only a fraction of the 5.3million "losers" would be compensated.

According to the YouGov poll, the Conservatives are on 44 per cent, Labour is on 26 - down three - and the LibDems have 17.

The lead would hand the Tories a majority of at least 50 seats at the General Election.

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More worrying for Labour is that the poll - which was taken in the wake of the revolt over Mr Brown's Budget plans - carries a gloomy message from voters ahead of crunch local elections next week.

The last time the Tories had such a big lead was October 1987, months after Lady Thatcher won a landslide third election.

But the survey was contradicted by an earlier one this week, which gave Mr Brown a glimmer of hope.

The ICM poll for the Guardian showed a Labour surge had slashed the Tory lead from 13 points last month to five.

It put Labour on 34 per cent, up five points, the Tories on 39 per cent, down three, and the LibDems down two on 19 per cent.

The contrast shows the volatility of the parties' fortunes, while the latest survey may just reflect the frustration voters vent on the incumbent party before elections.

Last night's poll appears to dash hopes that moves to reimburse millions of pensioners and low-paid workers hit by the abolition of the 10p rate would quell unrest.

Two-thirds of voters also said the Prime Minister had failed to give the Government a clear sense of direction.

Number Ten says Mr Brown has time to turn things around in the two years before the General Election, while a resilient economy could also help him recover his standing.

However, a meltdown in town hall polls on Thursday could spell the beginning of the end.

The survey for the Daily Telegraph came after ministers faced demands for more details of the rescue deal for 10p tax losers - forcing Chancellor Alistair Darling to plead for time to flesh out his plans.

Although Mr Brown has headed off a fatal Commons defeat on Monday, Labour is still expected to take a beating at the ballots next week.

"This entire humiliating U-turn is unravelling," Shadow Chancellor George Osborne said yesterday.

Under the compensation proposals women aged 60 to 64 will get extra cash through the winter fuel payment, while young low-paid workers will be reimbursed by tax credits and a higher minimum wage.

An estimated 5.3million Britons will lose an average £100 a year from the abolition of the 10p band.

Tories claimed fewer than two million of them would be compensated.

But the Social Market Foundation think-tank claimed only one in five victims would be helped if the Government kept to its £1billion cap on the cost of compensation.

It said making good the losses of the two main target groups would cost taxpayers more than £4billion.

Labour rebels last night warned the Government's pledges must be kept.

MP Lynne Jones said: "Gordon Brown and Alistair Darling know it would be foolish to betray the trust MPs have placed in them."

After Treasury Chief Secretary Yvette Cooper would not say if payments will be backdated, chief rebel Frank Field revealed Mr Brown had told him they would.

He added: "This is an agreement the Prime Minister put his stamp on. This package is now felt to be owned by the House of Commons."