LONDON (AFP) - Voting was under way Thursday in a key by-election in northwest England which is expected to deal a fresh blow to Britain's embattled Prime Minister Gordon Brown.

Three weeks after Labour slumped to its worst local poll results in 40 years, Brown could see the Crewe and Nantwich constituency seized by the opposition Conservatives for the first time ever.

The election, called after the death of the sitting Labour member of parliament, is being watched closely as a sign of how far support for the centre-left party's has slumped and the extent of the Tory revival.

It is also the first time voters have a direct say on the government's compensation package for the low paid, prompted by widespread outrage at its abolition of the lowest rate of income tax in April.

Polling stations close at 10:00 p.m. (2100 GMT), with the result expected early Friday.

Labour holds a 7,078 majority in the constituency -- in more favourable circumstances a comfortable margin -- but opinion polls have suggested there will be a sizeable "swing" to the Tories.

A ComRes poll for The Independent newspaper Tuesday put the Conservatives 13 points ahead, more than enough to overturn Labour's majority, while nationally Labour was 13 points behind the main opposition.

Other recent surveys have put Labour 20 points behind the Conservatives -- a record low -- with Brown criticised for the government's recent economic record and blamed for rising fuel and food costs and falling house prices.

A win for David Cameron's party would not only add to the pressure on Brown, around whom rumours of a leadership challenge have been swirling, but would be the Tories' first gain of a Labour seat in a by-election since 1982.

It would also be the Conservatives' first by-election win as an opposition party since 1978, and would be the first time they had held the constituency since its creation in 1983.

In addition to the psychological boost for Cameron after success in town halls in England and Wales on May 1, including the high-profile London mayor's job, a victory in Crewe would suggest his party can now win in traditional Labour areas concentrated in northern England.

Brown's predecessor Tony Blair's ability to win in the Conservative heartlands of southern England was key to Labour's landslide victory in the 1997 general election.

Even The Guardian newspaper, normally more sympathetic to Labour's cause, concentrated Thursday not on whether the Conservatives would win but the size of their victory.

"If they are to be serious, they need not just to break their 26-year duck of no by-election gain, but to do so in style," the centre-left daily's political editor wrote.

"Although an eight percent swing will be enough to take the seat, swings of 20 percent came to be expected from Labour in the mid-term of the (John) Major government."

An eight percent swing nationally would give the Tories a parliamentary majority if a general election were held now, according to the Electoral Calculus website.

Negative campaigning has been the feature of the by-election, with Labour and Tory activists at loggerheads over the credentials of their respective candidates and suitability to represent a working-class area.

Labour has scorned the Tories' Edward Timpson, a privately-educated lawyer, as too much of an upper class "toff" to understand the needs of an area famous for its railway interchange, carriage works and car factories.

The Conservatives point to the fact that Labour's Tamsin Dunwoody was parachuted in from her country home in Wales to take over the seat her mother Gwyneth -- herself from a notable political dynasty -- held until she died in April.

Party sources said the Conservatives would seek to capitalise on a victory in Crewe by calling an early election in Henley, the affluent seat in Oxfordshire due to be vacated by the new mayor of London, Boris Johnson.

The sources said that election could happen between June 26 and July 3.

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