The verdict from the Commons Committee on Standards and Privileges, that the visit should have been declared in the Register of Members' Interests, revived Labour embarrassment over last year's Formula One affair.
The Conservatives immediately condemned Mr Blair's "rankest hypocrisy", and demanded an apology from the Prime Minister in the Chamber of the Commons; a form of parliamentary punishment by ordeal that had not been proposed by the Standards and Privileges Committee.
But some senior MPs - Tory as well as Labour - last night turned on the committee, saying they would now feel compelled to register anything and everything, no matter how minor. One Labour critic said: "The whole machinery of Nolan was set up to catch rogues and rascals - not this tat and trivia."
But the committee issued a further warning to all ministers and MPs. "This is not the first case of its kind," it reported, "and we draw the attention of the House to the importance of registering, or seeking the Registrar's advice, in any case where there may be any doubt."
Mr Blair told the committee that he had not registered the Silverstone trip because he had gone "as part of my duties as leader of the Labour Party, not as an opportunity to watch motor-racing - hence my departure soon after the start of the race".
However, it was conceded yesterday by Mr Blair's official spokesman that during the visit, he had met Formula One's Bernie Ecclestone, who was later to make a donation of pounds 1m to Labour - leading to last year's controversy over government policy on tobacco sponsorship of the sport.
The judgement of Sir Gordon Downey, the Parliamentary Commissioner for Standards, who acts as Westminster's special investigator, was that the visit should have been registered, and the committee unanimously upheld his view. "The rules applying at the time," Sir Gordon said, "required the registration of any hospitality exceeding pounds 215 in value given to a Member or a Member's spouse, which in any way related to membership of the House."
Andrew Robathan, the Conservative complainant, said the "cost" of the hospitality would have come to pounds 600 for Mr Blair and his wife Cherie. Sir Gordon upheld that view - "a value of over pounds 300 per head" - adding that six other MPs who had been at Silverstone on the same day had registered the perk.
Sir Gordon said in his report that while there was "undoubtedly some confusion" over the principle of visits undertaken in an official capacity, Mr Blair was "mistaken".
The Prime Minister's official spokesman suggested the absurdity of the latest position, saying: "Every time William Hague goes to an agricultural show, does he have to declare the fact that it would have cost him pounds 60 and, if he took his wife, pounds 120?
For the Tories, Francis Maude said: "Tony Blair is caught in a trap of his own making. While sermonising about the need for all politicians to comply with the rules both in letter and in spirit, he was breaking the rules himself. It is the rankest hypocrisy ..."
Leading article, page 22Reuse content