Gordon Brown is planning to give away billions of pounds to low-paid families in the run-up to Christmas in an attempt to lift Britain’s recession-hit economy.

Speaking at a G20 financial summit in Washington, which endorsed action to ease the economic pain caused by the credit crunch, the prime minister said the case for tax cuts was now “unanswerable” and they would “show results as quickly as possible”.

He also made it clear that the less well-off would be the main beneficiaries of any fiscal stimulus announced in next Monday’s prebudget report, implying that those on higher incomes will receive little extra cash. The extra money, which will be delivered mainly through tax credits, would “entrench the fairness agenda”, he said.

It comes as an opinion poll for The Sunday Times, carried out by YouGov, shows that the Conservative lead over Labour has slumped to just five points, its lowest in a comparable poll this year, mainly as a result of Brown’s handling of the financial crisis. The Conservatives are down two on 41%, while Labour is up three on 36%. The Conservative lead has halved from 10 points in the space of a month. In September, when the financial panic erupted, the Tories were 19 points ahead.

Brown’s personal rating has soared, climbing by 41 points in the space of two months. He will be hoping to build on that when Alistair Darling, the chancellor, presents the pre-budget report.

Aides at No 10 said the tax credit system would be the main mechanism by which the extra money would be distributed. Tax credits primarily benefit low-income families, with 6m households receiving them in all.

With the pre-budget report scheduled to take place only four weeks before Christmas, it will be a challenge for the Treasury to put extra money into taxpayers’ bank accounts in time for the holiday period.

However, Brown, who has had talks with Kevin Rudd, the Australian prime minister, is understood to have been impressed by a recent £4.5 billion Australian fiscal package targeting poorer families and pensioners, which had been dubbed the “Christmas giveaway”. Downing Street aides were confident that some of the extra cash planned by Brown could reach people by the year’s end.

The maximum income that a family can earn and still remain entitled to receive credits is £66,350 for households with a child under one and £58,175 for those with other children. Officials say 90% of families with children under 16 are eligible for credits.

Neither the Treasury nor Downing Street would be drawn on the size of the planned giveaway, which is intended to be temporary. However, officials have privately signalled that the figure could be about £15 billion, equivalent to 1% of gross domestic product.

If devoted entirely to cutting tax bills, this would amount to several hundred pounds for every eligible family. However, Darling is also expected to announce extra cash for transport infrastructure projects and new “green” tax breaks.

Business will welcome action to boost the economy but favours direct tax reductions. The Institute of Directors will tomorrow call for a £20 billion temporary boost for the economy, including a 3p cut in the basic rate of income tax and 4p off corporation tax.

In their final communiqué at the end of the Washington summit the 20 leaders endorsed Brown’s plan for co-ordinated worldwide tax cuts. They agreed to “use fiscal measures to stimulate domestic demand while maintaining a policy framework conducive to fiscal sustainability”.

Brown described the agreement as a “route map to reform”. “I believe we will see many countries following this lead in the next few weeks,” he said. “Today’s G20 meeting is historic. We have reached important conclusions about trade, about financial stability and about the expansion of our economies.”

The summit also pledged to reopen world trade talks but it put off detailed decisions about the regulation of the global financial system and the international banks.

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