Gordon Brown and Alistair Darling ruled out a U-turn over the abolition of the 10p tax band yesterday, as the Prime Minister flew home from the Nato summit in Bucharest to increasing backbench tensions.

Mr Brown went straight to a “progressive governance” summit at a Hertfordshire hotel where he will urge international leaders today to do everything to restart the world trade talks as a way of boosting economic confidence.

But with Parliament starting a two-week recess Mr Brown’s whips and ministers were working to contain a growing rebellion over the decision made a year ago to scrap the 10p band, effectively doubling tax to 20p in the pound for about five million people on incomes of under £18,000.

The Times has learnt that some MPs were stunned yesterday to receive text messages from the whips asking whether their support could be counted on for the passage of the Finance Bill, which implements the Budget in legislative form.

The move underlined the jitteriness of the whips at signs of increasing indiscipline in the ranks, with one minister last weekend criticising the Government openly for being out of touch and Gerry Sutcliffe, the Licensing Minister, attacking the tax rise on alcohol in the Budget.

Although the 10p move was in last year’s Finance Bill it will be confirmed in this year’s and the whips are clearly worried at the scope for a successful amendment being tabled.

The Treasury moved yesterday to scotch the idea that a rethink might be possible with sources suggesting that it would cost billions.

Mr Brown is standing firm, according to senior figures in Downing Street. One said: “You always get this sort of thing when there is a change to the tax system. The Prime Minister believes that single people and childless couples – those most likely to lose out – are benefiting from innovations like the working tax credit.”

But the problem is unlikely to go away. Greg Pope, the Labour MP who tabled a Commons motion that attracted the support of roughly 30 of his colleagues, believes that he secured an understanding that mitigating factors would be looked at.

After withdrawing the motion, Mr Pope, who is normally loyal to the party leadership, said: “I have been given assurances by senior ministers that they will look at its impact, especially on pensioners, some of whom are losing more than £200 at a time of rising fuel costs.”

Mr Brown was confronted over the issue when he addressed the weekly meeting of the Parliamentary Labour Party on Monday. Nia Griffith, a former ministerial aide who was among those to tackle him, said it did “not look good” to voters. “We should have woken up to it sooner and it would have been easier to do something about it earlier on,” she said.

Fabian Hamilton, the Labour Leeds North East MP, said that he feared that the change would hit young, single people without families.

Mr Brown, meanwhile, will concentrate his efforts this weekend in trying to secure greater international cooperation. It was still uncertain last night whether Thabo Mbeki, the South African Prime Minister, would arrive for today’s talks.

All week Downing Street has felt it unlikely that he would be able to come because of the delicate state of affairs in Zimbabwe.

Mr Brown telephoned African leaders yesterday urging them to tell Robert Mugabe not to cling to power if he had lost the presidential election. One call was to Yoweri Museveni, Uganda’s President, who is regarded by No 10 as a key player in the negotiations over Mr Mugabe’s future.

Whitehall is reluctant to be drawn into questions surrounding any deal, such as whether Mr Mugabe will be offered immunity from prosecution in the International Criminal Court. Sources close to Mr Brown point out, however, that Ian Smith, the last leader of Rhodesia, was allowed by Mr Mugabe to remain in the country after losing office.

One option that diplomats believe Mr Mugabe might be under pressure to accept is to stand aside from any run-off election to let another Zanu (PF) candidate to run against the MDC. It is unclear whether Zanu (PF) rules would allow such a substitution.

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