LONDON (AFP) - Gordon Brown marked his first year as prime minister Friday with a humiliating fifth place in a by-election that the main opposition claimed had put them on track to regain power.

A YouGov/Daily Telegraph poll suggesting Brown was an electoral liability and news that economic growth slowed in the first quarter of this year compounded Brown's woes, exactly 12 months since he took over from Tony Blair.

Although Labour was never expected to win the seat of Henley, southern England, the scale of the loss was a surprise, as it trailed in behind the Green Party and even the far-right British National Party.

Labour saw its share of the vote fall more than 11.5 percent on the last general election in 2005.

Its candidate mustered only 1,066 votes -- below the five percent threshold of the total number of votes for him to retain his 500-pound (630-euro, 993-dollar) deposit.

The main opposition Conservative Party retained the safe seat vacated by Boris Johnson when he became London Mayor on May 1 by a comfortable 10,116 margin over the Liberal Democrats, slightly down on its 2005 majority.

As Labour tried to put the best gloss on the result, Tory leader David Cameron said he was pleased they had picked up votes from the Lib Dems and described the result as "disastrous" for Labour.

"I think what we are seeing is that people who voted for all sorts of different parties -- including the Liberal Democrats -- are now looking at the Conservatives and saying, 'Yes, this is an alternative to the government that I can believe in'," he said.

But he refused to crow, adding: "We have got a lot of work still to do, but I think this is another good step forward for the Conservative Party."

Labour ministers admitted the result was "terrible", but blamed voter dissatisfaction on the effects of external pressures like the slow-down in the world economy, rising oil prices and increased fuel, food and energy costs.

Office for National Statistics figures showed Friday that Britain's economy grew by just 0.3 percent in the first quarter of 2008 -- the slowest quarterly rate since 2005. The data was revised down from 0.4 percent in May.

Labour has now polled poorly in three elections since May, losing to the Tories in local polls and a by-election in a formerly safe seat, putting the opposition on course for a return to power for the first time since 1997.

The YouGov poll Friday put Labour 18 points behind the Tories, with 61 percent of voters rating Brown as an electoral liability.

Some 67 percent of voters believe the Conservatives will win the next general election, which is due no later than May 2010, while just 16 percent believe Labour will be returned for a fourth term.

The Henley result also saw bookmakers William Hill cut the odds on Brown being replaced as Labour leader before the general election from 5/4 to 11/10 and slashed the price of him quitting this year from 5/1 to 3/1

The Tories were made clear favourites to win the next general election at 2/7 -- their shortest odds since losing power.

Brown has seen support haemmorhage over the last 12 months after initially being praised for his more serious-minded, considered approach to government.

The former finance minister's reputation for sound economic management took a damaging hit from the near-collapse of the Northern Rock bank and his government's botched income tax reforms.

The government has also faced accusations of incompetence after the repeated loss of sensitive personal documents, including most recently intelligence files on Al-Qaeda and Iraq.

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