Nearly half of borrowers remortgaging next year could be locked out of the best deals by falling house prices.

As many as 49% of homeowners who have taken out a loan since the start of 2007 have borrowed more than 75% of the value of their property, Financial Services Authority data revealed last week.

Most lenders restrict their best deals to those with a deposit or equity of 25% or more, meaning many borrowers would be unable to get a good remortgage deal and would be forced on to the standard variable rate (SVR). This could cost an extra £333 a month on a typical £200,000 home loan, if the SVR is generally two percentage points above the best fixes and trackers.

In some cases, homeowners may have to pay tens of thousands of pounds to repay debt to qualify for top remortgage deals.

The Bank of England warned last week that 1.2m people faced negative equity if house prices fell 25% from peak to trough as predicted by Nationwide and Halifax.

However, economists point to a wider problem that as many as 2.5m — or one in five of all borrowers — would be in the remortgage danger zone.

Lenders began to cut fixed-rate deals last week in response to falling rates in the money markets — but only for those with deposits above 25%.

Nationwide drew heavy criticism for raising trackers for those borrowing more than 75% by up to 0.40 percentage points — even though Libor, which in part dictates the cost of variable rate loans — fell 0.16 points to 5.84%.

George Buckley at Deutsche Bank said: “When it comes to the amount lenders will advance as a proportion of the property value, 75% is the new 90%. Some people are going to be in real trouble.”

Deals for those with 10% deposits are still advertised in the best-buy tables — for example, Britannia has a two-year fix at 6.14% with a fee of £999 — but the number of such loans available has fallen 80% since July last year, according to Moneyfacts, and brokers say they are becoming increasingly tough to get.

Aaron Strutt of Chase de Vere Mortgage Management said: “First Direct has a lifetime tracker at just 5.49%, but there is a 12-week delay and the lender requires higher income than many others. It’s also up in the air as to whether you’re ever going to fit the valuation.”

Abbey, the biggest lender of mortgages this year, now requires a deposit of at least 15% for all deals — and offers much better rates to those with a deposit of 25% or more.

Last week, HSBC, another of the biggest lenders this year, pulled its deals for mainstream borrowers wanting to borrow more than 90%. Halifax and Royal Bank of Scotland will advance loans of 90% — though these deals are unavailable through brokers, a sign banks want tight controls.

Here we show how to beat the remortgage timebomb.

Ask about your secret rate

Homeowners are being urged to check their deals, as lenders may have a “reversion” rate at the end of the deal that is better than its SVR. Those who took out a two-year tracker with Alliance & Leicester at 0.26 points below Bank rate will pay 0.99 points above Bank rate when the deal ends — or 5.49%, against an SVR of 6.94%.

Get your valuation sooner rather than later — and argue

Valuations are now routinely coming in at 30% below expectations.

Linda Rogers, 37, a property developer from south London, was told this year that her home was worth almost £1m. When she came to remortgage with Abbey, the lender’s surveyor valued the property at just £700,000. After she disputed the valuation, the firm agreed to a value of about £850,000. She had threatened to go to the Royal Institution of Chartered Surveyors.

Take care if you are becoming a landlord

Helen and Craig Shaw, 31 and 35 respectively, are letting their Buckinghamshire home, having been sent to work in America. When they approached the lender, they were told they would need to move to a “consent-to-let” deal and that these were available only if they had 25% equity.

While they had put down a 25% deposit on the property a year ago, price falls had cut their equity to 15%, meaning they faced paying about £40,000 off their loan to get the deal, and the rate would go up to 6.34%.

Halifax later retracted, saying consent to let was available at up to 85%.

Don’t bank on interest only

Thousands of lenders have taken interest-only deals to cut the cost of repayments, but lenders are cracking down. Cheltenham & Gloucester (C&G), for example, last week said it will no longer allow new borrowers with deposits of less than 25% to make interest-only repayments.

No landline could count against you

C&G said: “Credit scoring has become tougher to pass over the last couple of weeks. Please ensure you input a home telephone number for your client if they have one, and full bank details as these small things are checked and can improve a score. It could be the difference between an accept and a decline at the moment.”