By Guy Faulconbridge and Kylie MacLellan
LONDON (Reuters) - British Prime Minister Theresa May's gamble on a snap election was under question on Thursday after a YouGov opinion poll showed her Conservative party's lead had fallen to a fresh low of 3 percentage points just a week before voting begins.
A failure to win the June 8 election with a large majority would weaken May just as formal Brexit talks are due to begin while the loss of her majority in parliament would pitch British politics into turmoil.
In the strongest signal yet that the election is much closer than previously thought, May's lead has collapsed from 24 points since she surprised both rivals and financial markets on April 18 by calling the election, YouGov data showed. Opposition Labour polled 39 percent against the Conservatives' 42 percent.
After a hectic campaign which was suspended by a suicide bombing last week, pollsters have offered a vast range for the result of the election: From May losing her majority to a landslide victory for her Conservatives of more than 100 seats.
Betting that she would win a strong majority, May called the snap election to strengthen her position at home as she embarked on complicated Brexit negotiations with 27 other members of the European Union.
But if she fails to beat the 12-seat majority her predecessor David Cameron won in 2015, her electoral gamble will have failed and her authority will be seriously undermined.
If May fails to win an overall majority, she would be forced to strike a deal with another party to continue governing either as a coalition or a minority government.
That would have uncertain consequences for Britain's $2.5 trillion economy, and future government policy on everything from government spending and corporate taxation to bond issuance.
Sterling weakened against the dollar late Wednesday after the YouGov poll but was trading at 1.2862 early on Thursday, 3 cents above where it was trading at the start of the campaign.
The FTSE posted its best month of the year, helped by a weaker pound.
YouGov said May was still the most favored choice for prime minister, though her 43 percent rating is the lowest it has ever been. Labour leader Jeremy Corbyn is on his highest ever rating of 30 percent.
May, an opponent of exiting the EU who won the top job in the political chaos following the June 23 Brexit vote, will use a speech on Thursday in northern England to pitch her vision of Brexit.
The Financial Times came out publicly for May, saying she was the safer bet, though The Sun newspaper cautioned that May's campaign was far too defensive.
"Corbyn is selling a wonderland," The Sun said. "And even if his promises are practically impossible and economically catastrophic, set against the Tories' unnecessarily defensive campaign it's small wonder he appears to be picking up support."
May has sought to present herself as the only party leader able to make a success of Brexit, though she has given few details of how she will handle the negotiations.
May will also set out plans on immigration, one of the key Brexit concerns for businesses.
"Set free from the shackles of EU control, we will be a great, global trading nation once again bringing new jobs and new opportunities for ordinary working families here at home," she will say, according to quotes supplied by her campaign.
(Editing by Michael Holden; editing by Ralph Boulton)