One in five buyers have not completed more than six months after the tax break was announced, according to Rightmove.
The property site estimated 100,000 buyers look likely to miss out on the saving, when the temporary holiday ends on 31 March.
The number of buyers coming to the market increased at the beginning of February even though most will be too late to beat the deadline, Rightmove found.
However, fewer sellers are listing homes with numbers down 21 per cent year on year, meaning demand is outstripping supply and pushing up prices once again.
Asking values increased by 0.5 per cent this month, in an unexpected market bounce.
And visits to Rightmove were up 45 per cent in the first week of February compared to the same point last year, with home-hunters sending 18 per cent more enquiries.
Tim Bannister, Rightmove’s director of property data, said: “Last year the market was unexpectedly buoyed by buyers’ determination to move and satisfy their new lockdown-induced housing needs.
“We may well be seeing a continuation of that this year. Rightmove’s early 2021 buyer data shows that despite the imminent end of the stamp duty incentive, all of the key buyer metrics are ahead of early 2020, itself an active period as the market was boosted by the post-election ‘Boris bounce’.
“As well as the current lockdown motivating buyer demand again, the restrictions have also been a factor in limiting new supply, leading to some modest upwards price pressure.
“These are strong signs that new buyer demand is not facing a cliff-edge after the 31 of March. It remains to be seen if this momentum will be enough to make up for the removal of the stamp duty savings that are benefitting many buyers and have been adding a sense of urgency to the whole market.”
Estate agents echoed the trends described by Rightmove.
Aldo Sotgiu, managing director of operations at Arun Estates, said: “We’re seeing high levels of buyer activity and demand remains strong but the number of new listings coming to the market is a concern with many potential sellers preoccupied with the wider problems associated with the pandemic.”
Jeremy Leaf, north London estate agent and a former RICS residential chairman, added: “The property market is proving much more resilient than many gave it credit for.
“Even though the chances of beating the stamp duty deadline by the end of March are now probably slim at best, we’re seeing many buyers and sellers shrugging off lockdown restrictions and the prospect of further economic bad news.
“Although viewing and sales agreed numbers are not at the same level as their pre-Christmas frenzy, activity remains fairly solid. So many moves were brought forward that transaction numbers in the second quarter will inevitably dip.
“However, prices are holding up quite well, supported by the continuing shortage of stock at the right price and location, as well as reasonable availability of competitively-priced mortgages and even higher loan-to-value products.”