Eighteen million families will be left £150 worse off next year and in 2010 unless Alistair Darling extends last week’s emergency tax cut to compensate losers from his scrapping of the 10p tax band, according to a leading think-tank.

In findings that will ratchet up pressure on the Chancellor, the Institute for Fiscal Studies (IFS) says that well over half of all households, including millions of the poorest, will be left poorer over the next two years as last week’s one-off rise in tax allowances vanishes along with higher winter fuel payments and as national insurance payment rises.

The IFS turns up the heat on Mr Darling still more as it also finds that, even after the £2.7 billion compensation in last week’s mini-Budget, millions still face bigger tax bills this year from the end of the 10p income tax band, with 900,000 of the poorest families still losing out.

This is despite the institute’s estimate that the Chancellor is now taking away £5.5 billion less this year through tax, national insurance, tax credit and benefit changes. This is the biggest such package since the 2001 election year, it says. Yet it finds that the abolition of the 10p band still means that six million individuals under 65 earning between £6,635 and £13,255 will pay more income tax this year, even after the higher personal allowances put in place last week.

The blow to many is offset as most live with a partner who will gain more than they lose. However, the IFS estimates that, after taking this into account, about 900,000 families with an average income after tax and benefits of £11,800 are still left worse off by the Budget.

Among those who will still lose this year are 500,000 single adults under 25 without children, another 115,000 childless single people aged 25 to 55 and 140,000 childless couples aged 25 to 55. Mr Darling’s Budget changes mean that these people lose an average of £83 a year, with the poorest facing the biggest losses.

The already severe political headache for the cash-strapped Chancellor over his next move is made still more acute as the IFS tells him that millions more people will be turned into big Budget losers unless he finds the funds to make this year’s bigger personal tax allowance permanent.

Yet the IFS report, which is published today, underlines how Mr Darling has left himself severely boxed-in, with the Treasury’s finances already deep in the red and most options for a more generous PreBudget Report in the autumn and 2009 Budget likely to prove very expensive.

The IFS says that the Chancellor confronts a stark choice between allowing millions of people to lose from his next Budget, or letting public borrowing climb.

Higher borrowing would raise the spectre of big postelection tax increases, as well as leading the Treasury to breach, by up to £5 billion, its “sustainable investment” rule that caps public debt at 40 per cent of national income, the institute finds.

Robert Chote, the IFS director, said: “By announcing a big one-off increase in the personal income tax allowance, Alistair Darling has not only created millions of winners this year; he has created millions of potential losers next year.”

Mr Chote argued that the Chancellor was likely to be afraid to leave people worse off, so that public borrowing is likely to take the strain. But he sounded a warning that this would “further undermine the credibility of the Government”.