Homeowners are routinely seeing 10% wiped off the value of their properties in a matter of months in a sharp acceleration of the housing downturn — with some suffering downgrades of 30%.

Lenders are knocking tens of thousands of pounds off properties at the mortgage-valuation stage, forcing some buyers to stump up bigger deposits to secure their mortgage deals or face higher rates.

There are growing fears that the crackdown on valuations will put pressure on an already fragile housing market.

Nationwide building society is expected to offer little comfort this week when it announces further house-price falls in its latest market survey. Although the drop is not expected to be as large as May’s 2.5% fall, chief economist Fionnuala Earley admitted her original forecast of single-digit falls this year looked optimistic. “I’m reluctant to put a number on that . . . [but] there are downside risks to a single-digit forecast,” she said.

Mortgage lending for house purchases has already fallen 56% in the past year to a record low of just 27,968 in May, according to the British Bankers’ Association, as first-time buyers have been unable to secure finance. Last week, Bank of Ireland, which owns Bristol & West, pulled the last mainstream deal offering borrowers 100% of the property price — previously a lifeline for cash-strapped first-timers.

Mervyn King, governor of the Bank of England, also warned of continued market weakness last week. He said: “Any prospective purchaser can’t really judge the likely level of house prices and where they’ll settle. It would be a very natural response to pull back and to wait until the market’s reached a new equilibrium before coming in.”

About a third of mortgage applicants suffered markdowns in the first three months of the year, according to figures from online chartered surveyor E.surv.

Simon Tyler of Chase de Vere Mortgage Management, said: “I saw a down valuation of as much of 30% recently. The asking price was £445,000 and the valuation came back at £315,000 in north London. This is highly unusual, but you can expect to see a lot of variance between two valuers at the moment.”

Down valuations are putting further pressure on sellers who have already had to knock hundreds of thousands of pounds off asking prices.

Tony Grounds, 50, a writer, first put his home at Broxbourne, Hertfordshire, on the market for £2.4m six months ago but was forced to lower the price to £2m. He later approached estate agents Knight Frank who recommended he drop the asking price to £1.795m a few days ago.

The down-valuation trap could also be a problem for buyers who have already secured a mortgage deal. Ray Boulger of broker Charcol said: “When a property is down valued it may seem like good news for buyers as they can get money off the property, but it may also push up the amount they are borrowing relative to the value of the property.

“If they have already secured a mortgage, this could mean having to put up a bigger deposit or paying a higher rate.”

Kulwant Bola, 41, a self-employed caterer, fell victim to a down valuation and had to find £30,000 to cover her share of the mortgage on a home in Dulwich, south London.

An initial valuation by HSBC put the prospective home at £605,000 in November. She had already exchanged contracts when she found a cheaper mortgage with Abbey at 5.24%. Although Abbey valued the home at £525,000 she was bound to buy at £605,000.

“We were fortunate that we could go to family for the money, but we were shocked to find ourselves in this position at all. When you are shopping around for a mortgage you don’t consider the prospect of such wide valuations,” she said.

Boulger said: “Although she had already exchanged contracts we didn’t expect to be running the gauntlet of such a significant down valuation when we applied for the mortgage with Abbey.”

He added: “Valuation is an art not a science but 10% is a big differential, especially in this price range.”

Remortgage deals are also being scuppered. Simon Pounder-Smith, 40, and his wife, Nicola, 40, both police officers, had come to the end of their two-year fix with Halifax and applied to Nationwide to remortgage their four-bedroom home in Nottinghamshire. They believed the home was worth £230,000 and wanted to borrow £155,000.

They applied for a deal at 5.95%, which was available for loans of up to 75% of the value of the property and would have required monthly repayments of £993.

However, after instructing a drive-by valuation, Nationwide said the property was worth just £200,000, forcing them to take a mortgage at 90% of the property value as they would be pushed above the threshold for cheaper deals, usally 75%. This would have meant an increase in monthly repayments to £1,017, or an extra £1,440 over five years. They appealed to Nationwide who agreed the home was worth £230,000.