British Prime Minister Gordon Brown has suffered a major blow after his ruling Labour Party lost a crucial by-election in his native Scotland.

The Scottish National Party (SNP) reversed a more than 13,000 vote majority in Glasgow East to snatch Labour's 25th-safest seat in Britain, with questions likely to be asked of Brown's future as premier.

Labour has endured two previous by-election losses in recent months and seen support plummet to record lows in the opinion polls.

Brown himself has been under pressure because of the rising cost of living, his administration's recent economic record and his own leadership style.

Overall, John Mason of the SNP garnered 11,277 votes, a 26 per cent increase in the party's share of the vote compared to the last general election in May 2005.

Compared to the 10,912 votes for Labour's Margaret Curran, it is a 19 per cent drop.

Turnout in the by-election, sparked by the resignation of Labour's David Marshall on health grounds, was around 42 per cent, compared to 48 per cent in 2005.

Brown, born and raised in nearby Govan and barely a year into the job, would lose his own seat were the 22.54 per cent swing replicated in a general election, Britain's Press Association said.

In that scenario, Labour would be left with just one MP in all of Scotland, compared to 40 after the 2005 general election, according to PA calculations.

Defence Secretary Des Browne, finance minister Alistair Darling and International Development Secretary Douglas Alexander would all lose their seats.

Mason said in his victory speech that the impact of the result would be felt as far afield as Brown's official residence in London, and declared that "Labour MPs across Scotland will be quaking in their boots".

"This SNP victory is not just a political earthquake, it is off the Richter scale," he said.

"It is an epic win, and the tremors are being felt all the way to Downing Street."

Curran, meanwhile, urged her party "to listen and ... to hear the message from the people of Glasgow East".

The SNP's margin of victory was narrow enough for Labour to request a recount, which delayed the announcement of the result by around an hour.

Brown's future as Britain's leader may now be called into question, with Glasgow East becoming Labour's third by-election defeat in recent months, one of which was a safe seat in northwest England while Labour finished an embarrassing fifth behind the far-right British National Party in the other.

The centre-left party suffered its worst local election defeat in England and Wales for 40 years in May, while a recent ICM poll in The Guardian on Tuesday put Labour 15 percentage points behind the main opposition Conservatives nationally.

Alexander acknowledged while speaking to the BBC that the defeat was "clearly disappointing" for Labour, and a "notable victory" for the SNP.

Glasgow East, whose boundaries have been redrawn since the 2005 election, has been given a rare national profile in recent weeks, with much made of the predominantly inner-city area's acute social problems.

SNP leader Alex Salmond has noted that in one part of the constituency, male life expectancy is 53.9 years - 15.1 years less than in war-torn Iraq. Others point out that in the Gaza Strip, men can expect to live to 70.5 years.

Unemployment is well above the 5.2 per cent national average, while there are high rates of alcoholism and drug dependency.

Such issues were high on the agenda on the campaign trail, alongside local concerns about law and order and post office closures and national ones like welfare benefit reforms, housing tax, and high petrol and energy bills.