Tony Blair's promise to increase healthcare spending to match other European countries will be much tougher to fulfil than first thought - according to an influential think-tank.

The Prime Minister, under pressure over NHS performance, pledged on Sunday to boost overall healthcare spending to the EU average by 2005.

Existing figures suggested the average is 8% of gross domestic product (GDP) - a crude measure of the wealth of a country.

The UK's spend was calculated as 6.8% - well below the average and beaten by every country apart from the Republic of Ireland.

And that would mean the government eventually finding an extra £11bn in real terms year on year.

However, the Kings Fund, which provides independent analysis on healthcare issues, has recalculated the average - and found it to be 8.6% instead of 8%.

Even that 0.6% extra would add a massive £5bn to the bill - making it even harder to find the money without either raising taxes or imposing cuts on other government departments.

'Problems ahead'

Kings Fund health systems director John Appleby said: "Finding the money to close the spending gap will be problematic in the long term.

"Although the Treasury has predicted surpluses of around £11bn, such surpluses are cyclical and will not be guaranteed in the long run."

He described other ways of increasing the total spend - such as levying more patient charges, or encouraging the expansion of the private section with tax breaks - as "inequitable and expensive."

The calculations means that, to reach the average within the five year mark, £2.65bn extra in real terms would have to be added to the UK health budget each year.

Even the substantial investment programme in the NHS since 1997 falls more than £1bn a year short.

If inflation in the NHS runs at 2% a year, the total budget in 2005/6 would have to be at least £77.5bn - compared with £48.5bn currently.

Conservative leader William Hague has suggested on Tuesday that total spend should be increased by developing the private sector.

Countries such as France and Germany - which have more private provision - spend a far higher percentage of GDP than the UK.