Alistair Darling's dithering over stamp duty has “paralysed” the housing market, prompting a 20% fall in prospective buyers, estate agents claim.

They report that thousands of house sales have collapsed since the chancellor confirmed that cutting the tax was one of a “number of measures” being considered to boost the economy. With house prices already falling at their fastest rate for 17 years, the hints about stamp duty at the start of this month have encouraged buyers to shelve purchases in the hope that they can save tens of thousands of pounds by waiting until the autumn.

The Royal Institution of Chartered Surveyors (RICS) is calling for the chancellor to announce his intentions on stamp duty as soon as possible.

James Scott-Lee, the RICS’s policy specialist on stamp duty, said: “In June and July we thought we had reached the bottom of the market. Then the government goes and does this and makes things even worse. August was expected to be bad, but the general view among agents across the country is that they are seeing 20% fewer new buyers in the market than they had expected at the start of the month.”

The National Association of Estate Agents (NAEA) said the fall in new buyers estimated by RICS was in line with reports from its members. More than a quarter of the NAEA’s members had seen sales fall through as a direct result of the uncertainty over stamp duty.

Peter Bolton King, the association’s chief executive, said the number of sales that had collapsed in the past three weeks over the issue would be measured by the thousand. “We urgently need clarity to get the market going again,” he said. “We cannot wait until the chancellor’s autumn statement. It is proving damaging not just to the housing market but also to the wider economy.”

Moveme.com, a website that allows users to manage their property online, said 63% of buyers who had put in offers in the past three weeks were now seeking to defer completion by 16 to 20 weeks in the hope of avoiding the stamp duty. “There is no doubt the market has been paralysed by talk of a [stamp] duty cut,” said a spokesman for the website. It claims that it normally has up to 20,000 customers a month.

The chancellor is believed to be considering a range of amendments to stamp duty. One possibility is to exempt all property sales from stamp duty, as the Conservatives did for eight months starting in December 1991. Another option would be to allow buyers to defer paying the tax. Other options include reducing stamp duty for first-time buyers or for those buying properties worth less than £250,000.

Frances Dore, a sales director from London, was on the verge of buying a £300,000 flat in Chiswick until she heard that she might be able to avoid paying the tax if she waited. “If it was not for the confusion over stamp duty I’d probably be in that flat now,” said Dore, 35. “But why on earth would I want to buy a property now, if I can hang on a few months and save myself £9,000?” Dore has told the vendors she will not complete until after the chancellor’s statement, unless they lower their asking price by £8,000.

Many homeowners are delaying putting their property on the market. Charles Wasdell, 35, a marketing manager who lives near Banbury, Oxfordshire, had planned to put his home up for sale early this month. “Our home is worth just over £500,000 – so the prospect of saving £20,000 of tax is enough to deter any buyer,” Wasdell said. “I don’t feel we’ve got much choice but to wait until next year.”