George Osborne has vowed to push ahead with £6bn of spending cuts despite some badly needed positive news on the public finances that showed the country's budget deficit was much smaller than first feared last year thanks to a windfall of taxes from the financial sector.

The Office for National Statistics said a surge in tax receipts at the end of the financial year had prompted it to revise down March's public sector net borrowing – the gap between the Exchequer's tax take and its spending – by £5.5bn.

That and other revisions left the deficit for the year as a whole down by more than £7bn on previous reports to £156.1bn, excluding the cost of interventions to support the financial sector. That is almost £11bn lower than former chancellor Alistair Darling's final budget forecast of £167bn for the 2009-10 financial year.

The Treasury stressed that the deficit was still the worst since records began at the end of the second world war. Much of the end of year uplift came from one-off tax effects as people rushed to sell shares or were given share options by employers ahead of a 50p tax rate for high earners in April.

A Treasury spokesman said: "The chancellor has made clear that tackling the deficit must start now, which is why £6bn of savings this year will be set out on Monday. It is welcome that borrowing has come in lower than expected for last year due to a windfall on tax receipts, but borrowing in April and for last year was still at record levels, which is why we need to act now to cut the deficit."

There was a slightly stronger than expected start to the new financial year too, with the net borrowing for April coming in at £9.96bn, the highest April shortfall on record but below economists' forecasts for £11bn. Analysts noted that it was £1.1bn worse than a year earlier, but the pace of deterioration in the public finances was slowing.

Alan Clarke, UK economist at BNP Paribas, said: "To put that into context, the pace of deterioration over the last year has been on average around £5bn compared with the same time a year earlier and as high as £10bn worse at times. Hence this outcome and substantial downward revisions to borrowing for the previous month are encouraging."

Against the backdrop of financial markets that are increasingly jittery about all countries' fiscal health, many economists echoed the Treasury's view that cuts were still needed despite the revised figures.

"This rather puts all of the pre-election bickering about the Conservatives' proposed £6bn cuts in the deficit this year into context. But it does not mean that a major fiscal tightening is no longer needed ... A huge fiscal squeeze is still coming and we still think that the budget on 22 June will include plans, at least, for sizeable tax increases," said Jonathan Loynes at Capital Economics.

Business group the CBI said there was no room for complacency and that the newly set-up Office for Budget Responsibility could well come out with a much bleaker outlook for the public finances.

The TUC said borrowing was "far from being out of control" and cutting too soon could derail a recovery in jobs and economic growth. "With the economy still fragile, the new government would be wrong to put this at risk by introducing spending cuts now. Not only will this damage the economy but it could create an even deeper deficit," said general secretary Brendan Barber.

The other measure of the public finances, the public sector net cash requirement, came in at £8.85bn in April. That was also the highest April on record and worse than a forecast of £6.75bn.