Former Tory MP and spy writer Rupert Allason yesterday was threatened with prison after a high court judge found him in "serious contempt" for failing to reveal his assets and the true state of his personal finances.

In a damning ruling, Mr Justice Neuberger gave him a "last chance", warning that if he does not "come clean" within 18 days, he risked being sent to jail for a "significant time".

By then, said the judge, Mr Allason, 50, had to give an account of his family trusts, the share ownership and value of his two companies, Westintel Research and Westintel Ltd, and payments from books - which he writes under the pseudonym Nigel West - and other freelance activities.

The court order was imposed last year after Mr Allason failed to pay £200,000 in costs and compensation to the publishers Random House. In what the judge yesterday described as an "unmitigated disaster", the high court last year comprehensively dismissed his claims to authorship of a book, The Enigma Spy, the post- humous autobiography of John Cairncross, the"fifth man" in the Cambridge spy ring.

Then, the court awarded Random House its costs on an indemnity basis - the highest scale that can be imposed.

Yesterday Mr Justice Neuberger rejected Mr Allason's request for a gagging order preventing the media from disclosing any information about his personal finances, a house in Berkshire owned by a Panamian-based company, and Westintel British Virgin Islands, which owned his Porsche.

Mr Allason claimed he received no income from Westintel Research, although he owned shares in that company as well as Westintel Ltd. Asked what the income of the first company was, he replied: "I have not the slightest idea."

Asked what was the source of income in one of his bank accounts, he said: "I have no idea."

His finances were organised by his agents, accountant and bookkeeper, he said. The court heard that Mr Allason had applied for an Individual Voluntary Arrangement - a way of avoiding bankruptcy.

That indicated his assets were nil and that he owed creditors over £1m. However, Mr Justice Neuberger said that the statement he made about his assets in applying for the IVA were also inaccurate.

The judge, who described Mr Allason's behaviour as "wholly unsatisfactory", said that if he refused to comply with the order next time his "feet won't touch the ground".

Stephen Shotnes, Random House's lawyer, welcomed the ruling. "It would be rather nice if Mr Allason gave the publisher what he owes them," he said.

Mr Allason, who described the publisher's attitude as "oppressive and bullying", told the court he was about to go to the US for research "and to see the FBI".