Polling entered its closing stages in a by-election on which much of Gordon Brown's political reputation is riding.

If Labour lose Glenrothes to the SNP it would be the party's second Scottish by-election defeat in less than four months after the loss of Glasgow East to the Nationalists.

The constituency is in Gordon Brown's political back yard and losing would be a particular humiliation as he twice visited Glenrothes to campaign personally - the first time he has done so since becoming Prime Minister. His wife Sarah also campaigned in the constituency.

Losing Glenrothes would leave him as the only Labour MP left in Fife - Dunfermline and West Fife, another constituency which adjoins Mr Brown's, fell to the Liberal Democrats in 2006.

For the SNP. taking Glenrothes would be a spectacular victory. It would boost their number of Westminster MPs to eight, taking them close to their record tally of 11 MPs in the mid-1970s. It would also be the first time that Nationalists have made two successive parliamentary by-election gains.

But if Labour hold the seat in what is expected to be a close result, comparisons could be drawn with the Glasgow Garscadden by-election of 1978, when Labour stopped an SNP advance. On the wider front it would vindicate Mr Brown's decision to campaign there and bolster his leadership of Labour after the knock this received at Glasgow East.

Labour sources said turnout was brisk, with some polling stations seeing up to 150 voters in the first hour of polling. But the party was making no predictions on the outcome. "We are not predicting victory, not predicting defeat - it's for the voters to decide," said a spokesman. We know Lindsay Roy is an outstanding local candidate and we believe that will reflect well in (the) vote."

The SNP said turnout was estimated to be in the 15-20% range at lunchtime. They made no public comment but an SNP source said: "We are in as good a position as we were in at this stage in Glasgow East." The SNP's Peter Grant, the leader of Fife Council, voted early with his wife Fiona.

Labour is defending a majority of 10,664 at Glenrothes - but the SNP overturned a bigger deficit to win Glasgow East. The seat, which fell vacant with the death of Labour MP John MacDougall, is also contested by the Liberal Democrats and the Tories and four other candidates.

Eight candidates are contesting the November 6 by-election: Morag Balfour (Scottish Socialist Party); Maurice Golden (Scottish Conservative and Unionist); Peter Grant (Scottish National Party); Louise McLeary (Solidarity - Tommy Sheridan); Jim Parker (Scottish Senior Citizens' Unity Party); Lindsay Roy (Scottish Labour Party); Kris Seunarine (Ukip); Harry Wills (Scottish Liberal Democrats).

-Nova