A Number 10 advisor has warned that key NHS targets may not be met despite the extra cash being poured into the health service.

Professor Michael Barber, head of Downing Street's delivery unit, wrote a confidential report for the prime minister and health secretary Alan Milburn, which was revealed in Wednesday's FT.

In the report Barber warned of the "immense risk" that the NHS will miss key productivity and waiting time targets in time for the next general election, expected to be in 2005.

This is despite the huge cash increase for the NHS being delivered as part of the ten-year plan for the health service.

Number 10 defended his comments, arguing that urgently-needed reform would only happen if the government kept up the pressure and it was a "sign of strength" that problems were being "maturely discussed".

"We do not reform the public services as important as the NHS by sending self-congratulatory memos across Whitehall," said the official spokesman.

"What this shows is that the government is not complacent. What we need is more reform not less reform.

The issue is particularly sensitive for Labour as from April this year voters will see their wage packets hit by the rise in National Insurance contributions earmarked to fund NHS improvements.

In his report Barber warns that the organisational changes implemented by the Department of Health to accompany the funding boost are happening too slowly and hindering progress.

The "weakness" of the primary care trusts and the strategic health authorities Milburn created last year are among the "very significant challenges" facing the NHS, the report says.

And the Delivery Unit, which monitors progress towards manifesto and other government commitments, is also concerned about the pressure of time.

Reform is "being managed to a very tight timetable" they warn before adding that the Department of Health has lacked the strategic leadership needed for the scale of change being promised.

"We are concerned that there has not been enough forethought and planning for this one-off opportunity...the risk is immense," they say.

The department needs "to get the right people into the right jobs faster" Barber argues.

The crucial target is the promise that waiting times will no longer than six months by 2005.

But "meeting the 2005 target will require a different approach - which is not yet tested - and substantial activity growth" the report advises.

Labour politicians have played down the significance of the report, saying that Barber and other Downing Street figures are employed to keep departments on their toes and ensure that they meet their priorities.

David Hinchliffe, the chairman of the Commons health select committee, told the BBC that the memo should not be "overstated".

But he accepted that many of the points made in the report rang true.

"There are some worrying points in respect of the scale of organisational change at the time of this very welcome investment," he said.

"If we are giving huge new investment - which of course we are at the present time - and we are not seeing the results, then I would be sharing that concern."