It is offering homebuyers an interest-free loan up to 25% of the value of a property for up to 10 years. If the prices have fallen when you want to sell, it will demand repayment of only 25% of the value at that time rather than the original value of the loan — meaning you will owe less than when you started.

Conversely, if house prices rise, you could end up owing more than the original value of the loan.

Buyers take out a standard mortgage on the remaining 75% of the value of the property from lenders including Halifax, Nationwide and Woolwich.

This means you can secure a better mortgage deal as you are in effect putting down a larger deposit. Nationwide, for example, offers 7.05% on a three-year fix with a £898 fee if you have 5% deposit. With 25% you can get a rate of 6.35%.

The deal, which is available on up to 100 of the firm’s 500 or so developments dotted around the country, comes amid fears that house prices will fall by 10%-20% by the end of next year, according to some analysts.

Taylor Wimpey said: “We believe that this is exactly the kind of scheme that will help first-time buyers get their foot on the property ladder.”

The 25% loan will only have to be paid back if you decide to move house. If house prices fall, you will pay back less.

For example, if you bought a home valued at £200,000, you would be lent up to £50,000. If it had fallen by 10% to £180,000 when you sold, you would only need to pay back 25%, or £45,000. If house prices increased, however, you would pay more.

Though you pay nothing on the loan until you sell or come to the end of your mortgage deal, you will have to pay interest at 2% over Bank rate after the 10-year period.

It is advisable that you have a savings plan in place to pay off the loan unless you are prepared to sell the property.

The deal is similar to a government-backed scheme which offers first-time buyers an interest-free loan on up to 40% of a property.

The Ownhome plan, which is in conjunction with the Co-operative bank and Places for People housing association, offers an equity loan of between 20%-40% of a property. The remainder has to be paid for through a Co-op mortgage on a repayment basis.

The deal is available to first-time buyers with a maximum household income of £60,000. The property being bought must also be for entirely residential purposes and be the buyer’s main home.

It must also be “immediately habitable”. This means that there cannot be any tenants in the property, and it must be in a fit state to live in.

The schemes have traditionally been available to key workers only but because of the housing crisis, the chancellor announced in the March budget they should be extended to all first-timers.

The equity loan is interest free for the first five years. After this, you pay a rate of 1.75% on the loan for the next five years, and 3.75% thereafter.

Customers can repay all or part of the loan at any time. The amount they pay will be calculated on the value of the home at the time of payment. There is no early redemption or payback charge.

Unlike the Taylor Wimpey deal, you have to pay back the full loan, rather than a percentage, if you decide to sell within three years. The maximum full Ownhome package that could potentially be lent is £435,000. This includes a £270,000 mortgage (62%) and a £165,000 Ownhome loan (38%).

The Co-op offers a lifetime tracker mortgage at Bank rate +1.04%; a three-year fix at 5.84% and a 25-year fix at 6.69%.