Help to Buy scheme blamed for housing bubble in capital, which has seen double-digit growth in prices over last two years

Soaring property prices spreading out across the capital from central London are stoking fears that families will be forced out of the city, with damaging consequences for its social fabric.

Research by Hometrack, a firm of independent property analysts, reveals that many of the capital's postcodes have seen double-digit growth in house prices in the past couple of years, with properties now worth more than they were at their peak before the crash of 2008. The figures have prompted concerns that government initiatives to encourage mortgage lending could stoke a boom that many experts already fear is out of control.

Hometrack's research suggests that the top 10% of London's most expensive properties are now valued at 23% above their 2008 peak. Those in the top quartile are 16% above their 2008 peak, while those in the second quartile have risen by 2%.

Significantly, it is postcodes beyond prime central London that are racing ahead. Hometrack's data indicates that properties in the east London districts of Hackney and Clapton are valued 27% above the peak before the crash, while in Lambeth the increase is 30%.

Further evidence of rising property prices is expected to be confirmed this week when the latest quarterly data is released by the Office for National Statistics.

Experts have raised concerns that the government's Help to Buy scheme, which was introduced in April and provides equity loans for first-time buyers of up to 20% towards the cost of new-build properties up to the value of £600,000, is fuelling a boom that will turn to bust. Organisations such as the International Monetary Fund have criticised the scheme, which will be expanded in January, making the loans available to all buyers and all types of property up to the £600,000 limit.

"This is throwing fuel on the flames," said the Liberal Democrat former Treasury spokesman Lord Oakeshott. "It's ramping up demand and doing nothing about supply. It's dangerous."

"By pumping money into the mortgage market without boosting the supply of new homes, Help to Buy can only make things harder for first-time buyers," said Campbell Robb, chief executive of Shelter. When these guarantees end, borrowers could find themselves trapped as mortgage prisoners unable to get an unassisted loan.

Shelter said that rising prices in the capital were pricing out families. Research commissioned by the charity suggests that across London as a whole, couples who start a family in their 20s will be saving for a deposit for 20 years – nearly double the time faced by childless couples.

In some cases this could mean their children will have left secondary school before they own a home, a prospect that gives them little alternative but to migrate out of the capital.