The boom in property prices is close to becoming unsustainable and the chairman of the UK's largest network of independent estate agents has warned that "fingers could be burnt".

TEAM's latest national house price survey shows that the average price of a home was £139,718 in mid-May - a 3.6% upwards jump from April. As usual there are regional variations, with increases in excess of 4% month on month being reported in five regions, Cornwall, Devon, Dorset & West Hants, East Yorkshire and South Wales.

Philip Muzzlewhite, TEAM's national chairman, said: "When we went against the trend at the start of the year, predicting a continuing rise in prices, we did not expect this accelerating trend. The market is on the brink of unsustainable growth. Care must now be taken else fingers will be burnt."

Jim Atkins, immediate past president of the National Association of Estate Agents, who is a TEAM agent based in Dorset, said: "With sustained low interest rates I would not forecast anything more than a slowing down in price appreciation as we move through the year, and certainly no bursting bubble."

Nigel Ellis, chairman of TEAM's North London and Hertfordshire branch said a reluctance to pay stamp duty was discouraging people from moving up-market. Typically in North London buyers were paying between £20,000 and £30,000 in stamp duty. In some areas more people were staying put and a shortage of homes being put up for sale was driving up prices. He said: "At the bottom end of the market you cannot get a two-bedroomed flat for under £150,000 unless it's ex-council or there is something wrong with it. At the top end we have just broken the £1m barrier in Crouch End for a detached Victorian house that was going for £900,000 only a short while ago."

Below is a region-by-region breakdown of "under offer" prices agreed on homes being sold through members of the TEAM network of independent estate agents. Each figure represents the average price agreed in the previous three months.