Homebuyers hoping to bag a bargain in a falling market by “gazundering” a property’s seller were dealt a blow yesterday, as an estate agent introduced emergency measures to prevent the unethical practice.

In a falling housing market, one of the greatest risks to homeowners trying to sell is that they might be “gazundered” by their buyer. Gazundering, the opposite of “gazumping” - is when a buyer demands a last-minute price cut right before contracts are exchanged, forcing sellers to accept amounts thousands of pounds lower than the original offer.

But yesterday, vulnerable sellers in the South East were offered a lifeline by Cluttons, the estate agent, which has cut its timeframe for sales in half to reduce the risk of gazundering. It will now demand that buyers complete a purchase within a deadline of three weeks from the date they make an offer, rather than the typical six, after seeing a rise in the number of buyers adopting the practice. Typically, buyers will try to knock the price down by as much as 10 per cent.

Cluttons said it had seen a fourfold increase in the number of sales being gazundered since the beginning of the market downturn. Twenty per cent of its sales are now subject to unscrupulous buyers, up from just 5 per cent before house prices began falling.

James Hyman, partner at Cluttons, said: “When the market was in the ascendancy, gazundering was practically non-existent. What we have done is a way of establishing whether a buyer is genuine.”

Under the new rules, a solicitor must be instructed the same day that an offer is made. A survey should take place within seven days, the mortgage approved in 14, and exchange completed within 21 days, with the agreed price being non-negotiable, regardless of the surveyor’s report. The previous exchange timeframe was anything up to six weeks.

Gazundering is more common in a market downturn because buyers are more conscious that they do not want to pay over the odds, and sellers who need to move are more desperate to sell before prices fall further.

Rebecca Williams, senior surveyor at Knight Frank, another estate agent, said: “Gazundering was rife during the recession of the 1990’s. It is widely viewed as unethical and immoral, but under English and Welsh law, it is not illegal.”

Any offer that is put forward is not legally binding until the exchange of contracts, so either buyer or seller can withdraw, demand an increase or reduce their offer.

However the percentage drop that gazunderers will attempt depends on the value of the home and the length of the conveyancing process. Sellers who allow delays in the process, for instance, by not having a home information pack available at the start of the exchange, are at most risk.

Ms Williams says: “There is no sure fire way of stopping this practice unless the Government rules that it is illegal. But sellers can take steps to reduce the risk.”

Times recommends

• Second homes: credit crunch casualty or money-spinner?
• How I built my own sliding house
• Top 10 incentives for house buyers