Focus: Poll upsets
Labour's devastating defeat in the Glasgow East ranks highly among the biggest ever by-election upsets.
In 1958, Mark Bonham-Carter gave the Liberals their first by-election gain since 1929 when he won Torrington, Devon, from Tories with a 219 majority.
In 1967, Winnie Ewing snatched a seat for the Scottish National Party, when she turned a 16,576 Labour majority at Hamilton into a 1,799 nationalist one.
In 1968, the Tories chalked up a 21.1 % swing to take Dudley on March 28 from Labour - along with Meriden and Acton.
This is believed to be the first time in British political history that a party has lost three seats at by-elections on the same day.
The following year, the Liberal Party captured Ladywood, their first Birmingham seat for 83 years, from Labour with a swing of 32%.
The Scottish National Party's Margo MacDonald gained Glasgow Govan from Labour on a 26.7% swing in 1973.
In 1976, the Tories won Walsall North on a 22.5% swing after the imprisonment of ex-Labour minister John Stonehouse, then won Ashfield with a 20.8% swing.
David Alton turned a Labour majority of 6,171 in Liverpool Edge Hill into a Liberal lead of 8,133, achieving a 32.4% swing in 1979.
Shirley Williams, a founder member of the SDP, won Crosby from the Tories on a 25.5% swing for the Lib/SDP Alliance.
In the last days of the Falklands War, the Conservatives took Mitcham and Morden off Bruce Douglas-Mann who had defected from Labour to the SDP.
There was a net 10.2% swing from Labour to the Conservatives on what was the last occasion the governing party took an opposition seat at a by-election.
Liberal Simon Hughes gained the highest swing since the Second Word War - 44.2% - when he won Bermondsey from Labour after an acrimonious campaign in 1983.
In 1988, Jim Sillars grabbed Glasgow Govan for the SNP on a 33.1% swing from Labour, emulating his wife Margo MacDonald's victory 15 years before.
The next year Labour's John Smith took the Vale of Glamorgan from the Tories after securing a 12.4% swing.
The Lib Dems captured Ribble Valley, among the top 15 Tory strongholds, with a 24.7% swing after a campaign dominated by the poll tax.
The biggest recorded swing against a government was 35.4% from Conservative to Lib Dems 4% at Christchurch in 1993.
In 2003, Labour suffered its first by-election loss since returning to power in 1997 when Liberal Democrats snatched Brent East on a 29% swing.
Labour would only have one Westminster MP north of the border if the Glasgow East by-election swing was repeated across Scotland at a general election.
With a 22% swing, the SNP would have at least 49 seats, the Liberal Democrats seven and the Tories one.
The swing would slice away the seats of Prime Minister Gordon Brown, Chancellor Alistair Darling and Cabinet ministers Des Browne and Douglas Alexander.