Ministers struggling to stave off defeat over the abolition of the 10p tax rate have been thrown a possible lifeline by an influential committee of MPs.

The Labour-led Treasury committee announced this afternoon that it is holding an inquiry into the impact of Budget measures on low-income households, particularly the effect of the 10p measure.

Importantly, it said it would finalise its conclusions before the report stage of the Finance Bill in the Commons, probably in June.

That means that MPs trying to avoid voting against the Government next Monday on Frank Field's 10p amendment have an excuse to wait. And it means ministers can say that they will take into account the Treasury committee report when drawing up measures for the pre Budget report.

But while getting the Government out of short-term embarrassment over next week's vote, the inquiry could pose a medium term problem. If it does indeed find that more than 5 million have lost out and proposes a series of measures to put it right then Alistair Darling could be put on the spot again.

He insists that the money is not available now to undo the Budget, and officials are saying nothing can be done until the autumn. Even so, it is unlikely that the committee would not have launched the inquiry without ascertaining informally whether it would be regarded in the Treasury as a possible route out of difficulties

The possible reprieve came as it emerged that 39 Labour MPs have already signed a rebel motion, tabled late last night, which challenges Gordon Brown's decision to scrap the 10p rate of income tax, setting the stage for what could still be a spectacular Commons showdown.

Given that Mr Brown has a majority of just 67, the rebels would have the numbers to deal him a damaging defeat.

The rebellion is being led by Frank Field, the former Welfare Minister. Other former ministers to have signed his motion include Kate Hoey, John Battle, Mark Fisher, Peter Kilfoyle, George Howarth and Janet Anderson.

John Cruddas, the former deputy leadership contender, and John McDonnell, the leftwinger who failed in his attempt last year to mount a leadership challenge against Mr Brown, have also signed.

The scale of the potential rebellion means that ministers and whips will still have to engage in furious lobbying before the amendment is voted on next Monday.

The Prime Minister himself was in conciliatory mood last night when he spoke to the Parliamentary Labour Party about the tax changes, promising he had listened to their concerns over the impact on the poorest workers and would “sort out” the problem by the autumn.

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