Even as the votes were being counted last night, questions were being asked
about the legitimacy of the result.

The first-past-the-post system is so stacked in Labour's favour that it
would be hard for Tony Blair to claim a popular mandate if, as the exit
polls suggested, Labour had won only 37 per cent of the vote.

The last time a party took power with less than 40 per cent of the vote was
October 1974, when Harold Wilson squeezed a majority of three out of
Labour's 39 per cent share of the vote.

The last party to win a substantial Commons majority on a lower share of
the vote was the Conservatives in 1922, when Andrew Bonar Law cruised to a
75-seat majority on 38 per cent of the vote, following the Liberal split.

Westminster's system has discriminated against the third-placed Liberal
Democrats for decades. Their post-election demands for proportional
representation will probably fall on deaf ears. But if, partly as a
consequence of rising support for Charles Kennedy's party, Labour and the
Conservatives come close in terms of share of the vote, the pressure for
reform could become irresistible.

Labour could have conceivably won a parliamentary majority on a smaller
share of the vote than the Conservatives.

"It would be a travesty of democracy," says Lord Lipsey, the Labour peer
and chairman of Make Votes Count. "How do you claim a mandate in that
situation?"

The first-past-the-post system has become progressively more favourable to
Labour since 1992.

In 2001 the party took 64 per cent of all seats with 41 per cent of the
votes, the biggest gap between the seat share and the vote share since the
second world war.

Theoretically, if the Conservatives and Labour both won 36 per cent of the
votes, Tony Blair would still have a majority of 50. It would require a
four-point Conservative lead to draw level with Labour in seats and a lead
of 9 to 10 points to scrape a majority over Labour.

According to the Electoral Reform Society, "this is probably the largest
degree of bias the system has ever produced".

The system is stacked in Labour's favour for several reasons.

Support for the Conservatives is spread more evenly across the country,
while the Labour vote is efficiently distributed, holding up where it
matters most. For example, between 1992 and 2001 Labour's vote in Liverpool
fell from 140,000 to 101,000, but it lost none of its five seats. In
Northamptonshire, the Labour vote rose from 119,000 to 132,000 and it took
five of the six Tory seats.

The 2005 Tory campaign has sought to correct this by ruthlessly targeting
resources on swing voters in the most marginal seats.

Boundary changes have not kept up with shifts in population. The electorate
in Labour's many urban seats is shrinking as more affluent (and supposedly
Conservative) people move to the suburbs and semi-rural areas.

This bias has been partially corrected by boundary changes in Scotland,
cutting 13 seats, 10 of them notionally Labour.

But in England and Wales, the rolling boundary review process is
cumbersome. The 2005 constituencies were last adjusted in any significant
way in 1994. The next boundary review will be completed in 2006, but is
based on the 2001 census, which means that the next general election will
be fought on 10-year-old population figures.

"It's like painting the Tyne Bridge," says Pippa Norris, a lecturer in
politics at Harvard University.

Boundary reviews are subjected to long consultation and political
negotiation. Expert assessment of the changes in the 1990s suggested they
would increase the 1992 Tory majority by up to 27 seats. But Labour
officials fought behind the scenes to protect their in-built advantages.

"The Tories were just dozy," Ms Norris says. "They did not understand the
significance of the changes."

Finally, turnout in recent elections has plummeted in safe Labour seats
while holding up quite well in marginal constituencies and safe
Conservative territory. This makes it easier for Labour to win seats for a
given share of the total national vote.

The problem of differential turnout is deep-seated, Lord Lipsey says, and
"cannot be corrected without changing the voting system".