Blair, who became prime minister on May 2, 1997 after winning the first of three straight general elections, will hold 10th place alone on Saturday when he overtakes Herbert Henry Asquith’s eight years and 244 days in power.

Fifty-one men and one woman have been called British prime minister and held the keys to 10 Downing Street, the PM’s residence in London.

Blair will also become the second-longest serving prime minister of the last 100 years behind Conservative titan Margaret Thatcher, who held the post throughout the 1980s.

He will also take seventh place outright in the league of continuous stints in office.

However, he is unlikely to top the 20-year record of Sir Robert Walpole, regarded as Britain’s first prime minister and the first to live and work at Number 10.

Blair has promised not to stand for a fourth term as prime minister at the next general election, due in May 2010 at the latest.

David Butler, the University of Oxford psephologist, told The Times newspaper that many long-serving leaders had been driven by their search for a place in history.

He said that Mackenzie King, the former Canadian prime minister of 22 years, was obsessed with topping Walpole’s record.

"I think people do take pride in length in office and it does affect behavior," he added.

"Mr. Blair may want to beat Baroness Thatcher’s 11.5 years, which would take him to November 2008, but I think it is rather vulgar to get involved in these calculations," he said.

Peter Hennessy, Professor of Contemporary History at the University of London, and author of "The Prime Minister: The Office And Its Holders Since 1945", said that longevity in Downing Street was largely down to "luck and caprice".

"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," he told The Times.

"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.

"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.

"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."

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, 174354, 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.