The UK's housing boom showed no sign of slowing down in April, with house prices rising by more than £100 a day.
The boom will "naturally cool", Nationwide says
Nationwide, the UK's largest building society, said UK house prices increased by 2.1% over the month - up from a 1.4% increase in March.
April's rise means the average house price in Britain is now £145,918, 18.9% higher than the same time last year.
The report is likely to add pressure on the Bank of England to increase interest rates when it meets next week.
Last month Nationwide raised its 2004 house growth price forecast to 15% from 9%.
House prices have already risen by more than 7% since the beginning of the year, but Nationwide said the market would "naturally cool" in the second half of the year as the impact of interest rate rises kicked-in.
"The pace of price growth we are expecting this year is clearly unsustainable given that affordability has become so stretched."
Figures from leading mortgage lenders show prices have surged at the beginning of the year after stalling towards the end of 2003.
But there is an emerging gulf between the optimism among lenders or estate agents and a growing band of pessimists in the City.
London fund manager Tony Dye - who earned the nickname "Dr Doom" for his stock market forecasts, including the technology market crash - has said the housing boom will "all end in tears".
He recently said he expected prices in London to sink by 30% in real terms over the next five years, adding "a national decline of the same order would not surprise".
His view that a fall is coming is shared by economists at institutions including Durlacher and Capital Economics.
With house prices surging, buying a property has become simply too expensive for many first-time buyers.
Those who argue that falls are coming say the ratio of average income to prices has climbed to an unsustainable level and must drop back.
Those arguing in favour of a "soft landing" for prices prefer to look at the ratio of income to the cost of servicing a mortgage - and on that basis, with interest rates relatively low, property remains affordable, they say.
Slow to erode
Nationwide said it was existing borrowers that were behind the market's momentum.
"It is existing homeowners, many with large amounts of equity as they purchased their property prior to the recent rapid rise in house prices, who continue to sustain the market by trading between themselves," said the Nationwide's Alex Bannister.
But he urged borrowers to take a "prudent approach" to the amount of debt they take on.
"In the current low inflation environment, real debt burdens will be slow to erode."