A report in The Times on Tuesday says the Foreign Office received memos which allegedly posed questions about the billionaire's business dealings.

Mr Hague, who was visiting Cheshire in the run-up to next week's Eddisbury by-election, said he had seen nothing to suggest Mr Ashcroft was unfit to be the party's treasurer.

'Flimsy' reports

The Times' report includes leaked documents which disclosed that diplomats told the previous Conservative government that rumours about Mr Ashcroft's business deals cast a "shadow over his reputation that ought not to be ignored".

Michael Ancram: Writing to cabinet secretary

But Conservative Party Chairman Michael Ancram, accompanying Mr Hague, said the report was part of a campaign being waged against Mr Ashcroft.

He said the documents were "flimsy and insubstantial" and he would be writing to the Cabinet Secretary, Sir Richard Wilson, to ask him to investigate the matter.

Mr Ancram said: "I believe today's story supports that political view and in particular the disclosure of the documents from the Foreign Office which under normal circumstances I think would be classified and unobtainable.

"So I think there are questions to be asked and I'll be writing to Richard Wilson to ask him to look into the matter and investigate how these documents came to be released."

The Times contains two documents, apparently written in 1996 and 1997, from officials commenting on Mr Ashcroft's attempts to expand his businesses abroad.

Mr Ashcroft has admitted giving the Conservatives £2m over the last two years.