Sir Peter Gershon has raised doubts about the ability of the Conservatives to deliver on their plans for £35 billion of public sector efficiency savings.

Giving evidence to the public administration committee, the author of the government's own efficiency drive said Tory plans drawn up by troubleshooter David James had failed to spell out how they would be delivered.

His comments will be seen as a blow to the plans put forward by Michael Howard and Oliver Letwin.

"The thing about the David James report is that it is totally silent on deliverability," Sir Peter told the MPs.

He said that his own £20 billion of efficiency savings were only achievable if ministers accepted his delivery recommendations.

James had "said nothing about how his recommendations would be delivered and therefore it is impossible to comment", Sir Peter cautioned.

He added that "to get to more than £20 billion you have to say something about deliverability - he hasn't said anything".

"I don't know whether it is achievable or not," he said.

The Conservatives have said they are able to save more money than the government by cutting back on some state functions.

Sir Peter's review, however, focussed on delivering existing services more efficiently.

In his comments to the committee, Sir Peter said he had considered going further in his own plans but expressed doubts about whether such moves could be achieved.

"I did consider some of the things, not all of them, that David James looked at," he said.

"One of the criteria I set myself was that nothing I recommended would be dependent on legislation.

"I believe some of the recommendations he has put forward require legislation to bring about, privatise the Met Office as an example.

"In a legislative programme... to plan on something like that happening by March 2008 you have almost got to guarantee the priority that you are going to give to that legislation and you are making all sorts of assumptions about the time to get that legislation through parliament.

"So whether that is achievable or not I don't know. There are parts of it potentially which you could do but I was very focussed on what could we plan on for March 2008, not in some longer time frame."

Plans defended

Responding to the comments, senior Conservatives insisted their plans were feasible.

"Sir Peter is quite right to say we would have to bring laws in such as abolishing regional assemblies, the New Deal, strategic health authorities and 168 quangos," said a party source.

"But this is clearly do-able in the lifetime of a Conservative government.

"They are simply a matter of political will."