The UK's housing boom is showing no signs of weakening, according to the Nationwide building society.

The mortgage lender has now admitted there is a risk its forecast of 15% price growth this year may be too low.

Its latest survey shows house prices rose by 1.9% in May, compared with a 2.1% increase in April.

The rise meant annual house price inflation hit 19.5% - the highest level since May last year. The average house now costs £149,020, Nationwide said.

At the start of the year the Nationwide was forecasting house prices would grow by 9%, but as the property market continued to soar it quickly raised this prediction to 15%.

Although it still expects the market to cool later this year - helped by higher interest rates - it now admits this 15% forecast may prove to be too conservative.

"Given the current momentum in the market there is now some upside risk to our forecast," said Alex Bannister, Nationwide's group economist.

Demand for property remained high, he said, with the fall in first-time buyers being offset to some extent by buy-to-let investors.

The supply of properties coming on to the market remained "constrained", he added.

However, Mr Bannister warned that the barriers facing first-time buyers and the high levels of personal debt in the economy meant the market could become becalmed in the future.

"There is a growing potential for a drawn out period of low activity and low price growth," he said.

But he said a fall in prices was unlikely in the absence of a recession or large increases in unemployment.

Recent rises in interest rates have been seen as an attempt by the Bank of England to cool the housing market, although the Bank has denied it is targeting property prices.

At its latest meeting earlier this month, the Bank's rate-setting body - the monetary policy committee (MPC) - decided to lift rates by a quarter percentage point to 4.25%.

It was the third rise in the space of seven months and it was subsequently revealed that the MPC members had discussed the possibility of a bigger rise.

Many analysts now expect rates to reach 5%, or slightly higher, by the end of the year.

However, the Nationwide said it expected a rate of 4.75% would be sufficient for the Bank to hit its inflation target.