I have two words of advice for Tony Blair now that he is 50 and has been prime minister for six years. Go now.

If he left office today, his place in history would be higher than that of most leaders during the last century, even those whose achievements at times during their periods of office seemed to be much greater.

Mr Blair would be remembered as the only Labour prime minister who won two general elections in succession each with thumping majorities. He never lost a by-election and had an almost unbroken lead in opinion polls.

Unlike all other Labour administrations, his has been in charge of a generally successful economy and this has enabled him to pump unprecedentedly large sums of money into services such as health and education.

He has also carried out social reforms such as introducing a minimum wage and schemes to reduce unemployment.

So why should he go now?

Enoch Powell said all political lives end in failure and there is only one shining exception to this rule: Harold Wilson who quit as prime minister 27 years ago when he was still at the top.

Commentators spent years trying to work out why the wily Wilson had gone. There was no subterfuge about his decision to go before things became worse.

Churchill came back for a second term as prime minister when heading towards 80 with failing physical and mental powers.

Attlee may have won two general elections but he lost another two and left Labour becalmed. Margaret Thatcher achieved necessary reforms but overstayed her welcome with the result now that she is far lower in public esteem than she should be.

Many other leaders felt they had plenty of mileage in their tanks until the electorate decided otherwise; among them John Major and Ted Heath. Even today almost 30 years after his two defeats in 1974, Heath still feels wronged and lives up to his nickname of The Incredible Sulk.

If Blair stays, things will only become worse for him. There are signs the economy is on the slide and that much of the money pumped into the public sector has been swallowed up by increased wages rather than better services.

The war in Iraq may have been won but it left a bitter taste in many mouths, among them MPs and party members.

Other issues such as foundation hospitals will strain their loyalty to breaking point and even a maverick leader like Blair cannot function without party backing.

There is a danger that Blair will see himself as indispensable; the man who can sort out any problems no matter how intractable, such as Northern Ireland or how unpopular such as putting Britain into euroland.

The problem for prime ministers is that their years in office, especially now they have so much security around them, tend to isolate them from the people.

If Blair made a promise to Gordon Brown all those years ago about quitting while ahead, now is the time to implement it and see if the Iron Chancellor has what it takes to go for the top job. I suspect that as in the case of Harold Wilson, Blair would be succeeded by an older man of great experience but with indifferent results.

Most of all I feel Blair needs a break. He has worked consistently hard under enormous pressure for many years. He is still young enough to take on another major job. Alternatively he could write his memoirs, which should be worth reading, or become one of the few politicians really able to spend more time with his family.