Mr Blair seized Churchill's record of eight years and 240 days, amassed during two stints in Downing Street, on Tuesday.

The Labour leader will become the second longest-serving Prime Minister of the past 100 years when he passes Asquith's eight years and 244 days' continuous service on the last day of the year.

While Mr Blair is unlikely to knock Sir Robert Walpole from his place as Britain's most enduring PM, having announced that he will not contest the next election, he may yet be tempted to push for one final record — beating the 11 years 209 days that Margaret Thatcher led the country between 1979 and 1990.

Mr Blair has vowed to serve a full third term but few expect him to pursue this literally until the end of 2008 — the point at which he would overtake the Iron Lady.

In the league of continuous stints at No 10, Mr Blair now comes in at seventh place since Walpole first assumed the title of Prime Minister in 1721.

David Butler, the Oxford psephologist, said that a concern for a place in history had driven many long-serving leaders. He said that Mackenzie King, the former Canadian Prime Minister who led his country for 22 years, was obsessed with beating Walpole’s record.

Mr Butler added: “I think people do take pride in length in office and it does affect behaviour.

“Mr Blair may want to beat Baroness Thatcher’s 11½ years, which would take him to November 2008, but I think it is rather vulgar to get involved in these calculations — you would be making wry comments in The Times if Mr Blair were to hang on just long enough to beat Thatcher’s longevity.”

Peter Hennessy, Professor of Contemporary History at Queen Mary, University of London, and author of The Prime Minister, said that longevity in political office owed more to “luck and caprice” than anything else.

But he believed that Mr Blair's legacy was, in part, already assured.

“It is always tricky even to begin to write the longer-term history of a premiership before it is over and properly laid to rest, yet one thing can be said with certainty about the Blair premierships: the remaking of the British constitution with devolution, the Human Rights Act and in a mild way the Freedom of Information Act, means that he is a prime minister who has presided over a profound and enduring change.

“It is matched only by the completion of the franchise [universal suffrage] which took from 1832 to 1948.

“Paradoxically, he rarely mentions this and yet if he decided to resign tomorrow this is the aspect of his premiership most historians 30, 40, 50 years on will seize upon — if you forget about the wars.”

Mr Hennessy added: “Longevity is as much as anything a matter of caprice and luck. Even the most advanced American spy satellite passing over London could not have detected a Conservative opposition of any kind until recently.”

The time a prime minister spends in office also seems to be reflected in his or her general standing.

A Populus Network poll of opinion-formers this year put Lady Thatcher far ahead of Mr Blair in terms of success in office.

She was rated as “very successful” by 55 per cent, twice the number putting Mr Blair into this category. However, taking account of those rating both leaders as “fairly successful”, Lady Thatcher was on 90 per cent, with Mr Blair on 83 per cent.

A survey of historians and political scientists of the most successful 20th century prime ministers last year by MORI and the University of Leeds put Clement Attlee top, Churchill second and David Lloyd George third. Lady Thatcher was fourth and Mr Blair sixth.

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Top ten longest-serving British prime ministers:

1. Sir Robert Walpole, 1721-42, Whig -- 20 years, 314 days

2. William Pitt "The Younger", 1783-1801 and 1804-06, Tory -- 18 years, 343 days

3. Robert Banks Jenkinson, Earl of Liverpool, 1812-27, Tory -- 14 years, 305 days

4. Robert Gascoyne-Cecil, Marquess of Salisbury, 1885-86, 1886-92 and 1895-1902, Conservative -- 13 years, 252 days

5. William Ewart Gladstone, 1868-74, 1880-85, 1886 and 1892-94, Liberal -- 12 years, 126 days

6. Lord North, 1770-82, Tory -- 12 years, 58 days

7. Margaret Thatcher, 1979-90, Conservative -- 11 years, 209 days

8. Henry Pelham, 1743-54, Whig -- 10 years, 191 days

9. Henry John Temple, Viscount Palmerston, 1855-58 and 1859-65, Whig -- nine years, 141 days

10. Herbert Henry Asquith, 1908-16, Liberal -- eight years, 244 days.