British house prices jumped by the biggest amount in more than four years last month, mortgage lender Nationwide said on Friday, in news that is likely to fuel concerns that government policies may be creating a housing bubble.

A 1.4 percent surge in house prices in December alone - the biggest single-month increase since August 2009 - means the average cost of a home is now 8.4 percent higher than in December 2012, according to the monthly report.

Prices are now rising at their fastest annual rate since June 2010, and both the annual and monthly increases are bigger than any forecast in a Reuters poll before the data.

Many economists are concerned that a government scheme launched in October which makes it easier for home-buyers to get a mortgage with only a 5 percent deposit is likely to raise prices, rather than boost construction as the government hopes.

Nationwide said that years of limited construction of new homes that pre-dated the 2008-09 financial crisis meant that even a small increase in demand was leading to price rises.

Other factors behind rising prices included falling unemployment and a generally brighter economic outlook, said Robert Gardner, Nationwide's chief economist.

"Policy measures also played an important supporting role by helping to keep mortgage rates close to all-time lows and improving the availability of credit, especially for those with smaller deposits," he added.

Prime Minister David Cameron said on Thursday that Help to Buy had led to 6,000 extra mortgage applications between October and December - a relatively small number compared to the more than 60,000 mortgages approved in a typical month.

This figure itself is well below the average monthly levels of 90,000 which were typical in the years before the financial crisis, and Nationwide said that house prices remained 5 percent below their peak of late 2007.

Nonetheless, price rises in some parts of Britain remain striking. Prices in London in the three months to December were 14.9 percent higher than a year earlier, making it the region with the fastest-rising house prices, and one where prices are now 14 percent above their pre-crisis peak.

Manchester, Britain's fourth-largest city, reported a 21 percent annual rise in prices, though northwest England as a whole only reported a 5.0 percent rise in prices.

The Bank of England has so far said it does not see signs of a bubble, but last month Governor Mark Carney warned that Britain's housing market had a tendency to heat up rapidly.

And in November the central bank said that as of the start of 2014, banks would no longer be able to access to a scheme that provided cheap mortgage finance, and instead would only get help to support business lending. – Reuters