Tony Blair has passed Clement Attlee's record to become the longest continuously serving Labour prime minister.

After six years and 93 days in Downing Street, Blair has broken a record few would have thought possible at the height of Thatcherism in the 1980s.

Then Labour was in the electoral doldrums and found it hard enough to govern itself.

That fact alone makes this weekend's achievement significant.

But as Blair himself has been the first to point out, that he can break Atlee's record with a tenure of less than seven years in office says more about his party's historical failures than his own success.

The current prime minister has yet to beat John Major's spell of six years and 155 days in Downing Street.

And he lags far behind the 20th century record holder Margaret Thatcher and further still behind the all time best of Sir Robert Walpole.

Thatcher's Conservative administration was in power from May 1979 to November 1990, a total of 11 years and 209 days.

However Sir Robert's Whig majority in parliament kept him in power for almost twice that length. Between 1721 and 1742 he served for 20 years 314 days.

While Blair claimed this week that his appetite for doing the job is "undiminished", he may have more realistic targets in his sights.

A full second term of government would see him overtake Harold Wilson as the Labour prime minister with the longest cumulative time in Number 10.

In two spells in the 1960s and '70s Wilson occupied the highest office for a total of seven years and 279 days.

Confusing the picture slightly is Ramsay MacDonald. The first Labour prime minister spent six years and 289 days in the top job.

But Blair has already overtaken MacDonald's period of time as a Labour leader of the government, as the party's great "traitor" spent four of those years at the head of a National administration.

Another milestone in his view could be the record of Herbert Henry Asquith. The Liberal prime minister, to whom Blair has often been compared, held office from 1908 to 1916.

Asquith's eight years and 244 days could see Blair into the mid-term of a third Labour parliament.

Having won a major landslide, set about radical reform of the constitution, redistributed wealth and talked of a new "radical century", Asquith eventually ran out of steam.

His government got bogged down in the grind of foreign conflicts - principally the first world war.

Eventually he was overthrown by his longstanding rival; a reforming chancellor and Celt.

The example of David Lloyd George bodes well for Gordon Brown, few prime ministers last much longer than Blair has already.