LONDON  - British Prime Minister Gordon Brown suffered another serious blow to his leadership Friday after his governing Labour Party lost one of its safest seats in a by-election in his native Scotland. The Scottish National Party (SNP) reversed a majority of 13,500 in Glasgow East to snatch Labour's 25th safest seat in Britain, prompting more questions about how long Brown can remain in Downing Street. But the premier insisted he could still win the next general election after David Cameron, leader of the main opposition Conservatives, called for a snap poll, and added he was "getting on with the job". "Whatever these difficulties... not only do we have the right policies, but when the time comes we will be able to persuade the British people," Brown said in a speech in Coventry, central England. Brown has until June 2010 before he has to call a general election. Despite only being in the job for a year, Brown is under mounting pressure from Labour lawmakers after a string of terrible results in by-elections, local elections and London's mayoral election. The loss of Glasgow East in Labour's traditional central Scotland heartland is being seen as even more embarrassing than that - one of the biggest political upsets of recent times. Questions are now being asked about how long Brown can stay as premier, even though there seems to be no obvious contender waiting in the wings. "Change of policy or change of job - that's the message of Glasgow East for Gordon Brown," said Scotland's First Minister Alex Salmond, leader of the SNP, adding that his party now "command(s) the agenda in Scottish politics." The result was a major boost to the pro-independence SNP, which has run the devolved government north of the border since last year and wants a referendum on Scotland breaking from London in 2010. But senior ministers and party leaders rallied round Brown. "I believe that Gordon Brown is the best Prime Minister. He is the best leader of our party," Chancellor Alistair Darling told BBC radio. The head of Labour's parliamentary party Tony Lloyd said "very few voices" within the party wanted a change of leader. "The answer to this isn't looking for Gordon Brown's scalp," he said. The victorious SNP candidate in Glasgow East, John Mason, won 11,277 votes, a majority of 365 over Labour's Margaret Curran, who suffered a drop of 19 per cent in vote share on the last general election in 2005. Turnout in the by-election, sparked by the resignation of Labour's David Marshall on health grounds, was a higher-than-expected 42 per cent. Brown, born and raised in greater Glasgow, would lose his own seat in Scotland were the swing replicated in a general election, Britain's Press Association news agency said. International Development Secretary Douglas Alexander, a key Brown ally, said voters were frustrated by global economic conditions, adding the premier should not shoulder all the blame for the result. "If you want me to say it is a bad result, it is a bad result," he told the BBC. "I don't think it is a night to say it is about one particular individual." Glasgow East is its third by-election loss in recent months - the first came in May at Crewe and Nantwich in northwest England, when Labour lost a safe seat to the Conservatives, while in June Labour came fifth behind the far-right British National Party at Henley in southern England. A recent ICM poll in The Guardian on Tuesday put Labour 15 percentage points behind the main opposition Conservatives across Britain.