THE SCALE of the protest vote by former Labour supporters disillusioned with Gordon Brown will determine who wins the Glasgow East by-election. Canvass returns from both Labour and the SNP point to a knife-edge contest on Thursday, decided by less than 1000 votes.

With only four full days of cam-paigning left, the sheer scale of those yet to make up their minds has been identified by the front-runners as holding the key to victory.

Despite a weekend poll for the Daily Mail putting Labour on 52%, 17 points clear of the SNP, the sample has largely been dismissed as "rogue" by both Labour and SNP campaign teams.

In 2005 Labour won 61% of the vote on a 48% turnout. The Mail poll, conducted by Progressive Scottish Opinion, would mean Labour support holding firm in Glasgow, a result undermined last night by yet another UK poll that showed Labour trailing the Conservatives by 21 points - the biggest advantage ever shown in the ComRes polls for the Independent On Sunday. The poll put Labour on 24%, the Tories on 45%, with the Liberal Democrats on 16%.

Of those questioned, 68%, including 38% of Labour voters, said they believed Labour would lose the next election.

However, if Labour lose in Glasgow this week, the combination of failing to hold Labour's 25th-safest seat, alongside a poll pointing to a three-figure Tory majority, will raise serious questions about the prime minister's ability to remain in office amid rumours of a potential autumn challenge to his leadership.

Campaigning in the constituency for the eighth time, First Minister Alex Salmond said his party's own private polling showed those "undecided" would determine the result this week. "This thing is there for us. Our own support is strong and motivated, and we've already made up the ground that the Conservatives did to win in Crewe and Nantwich," he said.

The SNP's private polling still shows Labour's Margaret Curran marginally ahead on 26%, with the SNP candidate, John Mason, four points behind. Labour's internal polling has also told them that the result on Thursday will be "tight as hell". However, the numbers of the undecided will worry Downing Street and the Labour Party in Scotland more than it will worry Salmond.

Mason indicted a subtle shift in the SNP's strategy, saying he would be emphasising to the undecided in the constituency that voting SNP was a safe option. He said : "I'm under no illusion about how difficult it is for people who've normally voted Labour giving us their vote. But this is a by-election, not a general election. The government will not change if they vote for me. But it will send a message that the government in London is out of touch and cannot continue as they have been."

Labour also appear to have re-focused their efforts on the undecided. Curran, who should have been upbeat given the Mail opinion poll, said she was "taking nothing for granted" and would continue to campaign as though she was fighting a Labour marginal and "fighting for every vote".

Stopping just short of criticising Labour's record in Glasgow East, she nevertheless said that if she won she would take the message to London that more had to be done. However she denied she was a government rebel-in-waiting and said that serially voting against the government achieved little.