By-election defeat in Eastbourne came a month before Thatcher's fall

Labour's loss of Brent East was its first by-election defeat since 1997.

That helped reintroduce the shimmer of excitement around by-elections seen when governments have less sizeable majorities than Labour's current margin.

Indeed, the current Parliament has so far seen only five by-elections in England, Scotland and Wales.

That compares with:

• 1997-2001 Parliament - 16 by-elections, with only one seat changing hands when the Conservatives lost Romsey to the Liberal Democrats

• 1992-1997 Parliament - 17 by-elections, with eight seats changing hands, all of them from Conservative defeats

• 1987-1992 Parliament - 24 by-elections, with seats changing between the parties eight times, all but one of which involved a Tory loss

• 1983-1987 Parliament - 16 by-elections, with five seats changing hands, four of them from the Tories

• 1979-1983 Parliament - 17 by-elections, with six changes between the parties, including three losses each for Labour and the Conservatives.

'Khaki' election?

By-election defeats are often seen as just something governments in mid-term have to endure.

But they can often have more serious consequences.

The Brent East defeat for Labour, one of its safest seats in the country, was interpreted as a sign of disaffection with the Iraq war.

The result also signalled the Conservatives' failure to make progress under the leadership of Iain Duncan Smith.

In John Major's premiership, the clutch of Tory by-election defeats were seen the as a symbol of the party's crumbling fortunes, as well as whittling away the government's slim majority in Parliament.

Thatcher downfall

But Mr Major entered Downing Street after another significant by-election result. Defeat in true-blue Eastbourne in October 1990 dented Margaret Thatcher's authority just a month she left No 10.

His accession did not prevent further Tory by-election defeats before his success at the 1992 national polls.

In the early 1980s, the rise of the Social Democrats was shown in by-elections.

Among the gang-of-four who left Labour to establish the SDP, Shirley Williams re-entered Parliament at the Crosby by-election in November 1981 and Roy Jenkins followed the next March.

Tony Blair will be anxious to ensure this week's by-election results are nothing more significant than a mid-term blip.