The index found that house prices are increasing strongly across most parts of the UK, with prices in London continuing to show the strongest growth.
London saw a record annual increase of 20.1% whilst l the South East (9.6%) and the East (8.6%) also reported strong growth.
Excluding London and the South East, prices increased by 6.4% in the 12 months to May 2014.
But Jonathan Samuels, chief executive of Dragonfly Property Finance, said the current levels of growth are unsustainable.
He said: "Annual house price growth of 20.1% is borderline insane, but there are signs that the capital started to cool slightly in June and July. I would expect next month's figures to show a less dramatic rate of growth.
"Things certainly have to calm down in the capital. 33.7% higher than the pre-financial crisis peak is mind-boggling.
"Remove London and the South East and annual house price inflation of 6.4% is much more stable and sustainable. These are words that London doesn't appear to recognise.
"With interest rate rises to come and mortgage advances more conservative, things may well come under control yet.”
