Newspapers have raised a series of charges against the billionaire businessman's dealings in recent days.

Mr Ashcroft, who is now a Belize national and its UN ambassador, has donated £1m to the Tories in the past year and is its single largest financial backer.

William Hague: "There is a lot of smear and lot of innuendo"

But Mr Hague insisted no substantial allegations had been put forward to lead him to question his treasurer's role.

"We're dealing with leaked memos and files from the selective administration that are concerned with things like whether he had a button on his sleeve.

"The only substantial allegations are about the last government helping him in some way with a tax treaty over Belize. As the Labour foreign minister said yesterday, they would have done the same for anyone.

"Nobody has actually made an allegation of wrongdoing in this case.

"There is a lot of smear and lot of innuendo and people enjoying writing about an unusual person who has a lot of business interests, but they ought to actually come up with an actual allegation if they want to continue it."

Mr Hague confirmed the size of the donations given to the Conservatives by Mr Ashcroft, but said the treasurer had worked hard to broaden the party's financial base.

"I want to see a large number of people giving a small amount of money and that's what he's working towards."

William Hague: "We are dealing with selectively leaked files"

The Tory leader attempted to throw back concerns over party funding to Labour.

"I'm surprised the Labour Party has the nerve to talk about this, because after Formula One and fox hunting there's only party where a large donation is coincidentally followed by a change in policy and that's in the government."

Michael Ashcroft is now a national of Belize

Mr Ashcroft himself has said he is "appalled" by what he described as a smear campaign against him.

He said: "I condemn this process of smear. I can firmly state that the conclusions they seek to draw from it are without foundation."

The basis for the row over Mr Ashcroft is two documents The Times published, written in 1996 and 1997 by government officials.

One alleged that Mr Ashcroft threatened to use local politicians to "stir up trouble" for the UK Government if he was not allowed to set up a bank in the Turks and Caicos Islands.

BBC Political Correspondent John Pienaar: "Labour have made him a target"

The other key charge is that he used his influence with the previous government to put pressure on Belize not to change its tax laws.

Foreign Office diplomats also warned about Mr Ashcroft's reputation in the leaked documents, as well as pointing out he appeared hung-over and had a button missing from his shirt.

The Foreign Office is currently conducting an inquiry into the leaking of the allegations.

The Times' editor Peter Stothard responded to Mr Ashcroft's charges, saying: "The Times has been doing nothing more than conducting a legitimate investigation into a senior and secretive figure in British public life."

Foreign Office minister Tony Lloyd confirmed on Wednesday that Mr Ashcroft had approached the Tory government about planned changes to the Belize banking laws which would have affected his interests.