The picture of the policeman at the door of No 10 Downing Street is one of the most iconic in the life of our country, on a par with that of the Beefeaters at the Tower of London.

Of course, the copper at the gateway to political power is there to protect the Prime Minister. But over the last few weeks, a police presence in Downing Street has come to mean something very different.

Twice Mr Blair has been interviewed by officers conducting the loans for honours inquiry. Meanwhile two of his closest aides have been arrested and interviewed under caution, for the second time in connection with a potential cover-up.

It may well be that Mr Blair is innocent of any wrong-doing. But in political terms, this spectacle is deeply damaging.

Mr Blair came to power vowing to clean up British politics. He promised to be "purer than pure" and to put an end to the sleaze that marred John Major's premiership. But after 10 years of scandals involving loans and donations, freebie holidays, passports and visas for friends and alleged favours for casino bosses, Mr Blair's fine words have a hollow ring today.

David Cameron rightly urged him to go last week.

I suspect that the verdict of history will be kinder to Mr Blair than that of his tormentors in the media today.

While as a Conservative MP, I know that the narrow interests of my party are best served by Mr Blair clinging on well after his 10th anniversary in power. But as a citizen, I fear he now diminishes Britain.