By Guy Faulconbridge and William Schomberg
LONDON (Reuters) - Prime Minister Theresa May could lose control of parliament in Britain's June 8 election, according to a projection by polling company YouGov, raising the prospect of political deadlock just as formal Brexit talks begin.
In stark contrast to opinion polls that have until the past week shown May on course for a big win in the snap election she called, the YouGov model suggested May would lose 20 seats and her 17-seat working majority in the 650-seat British parliament.
The YouGov constituency projection, based on 50,000 interviews over the course of a week, showed May would win 310 seats, down from the 331 seats won by her predecessor David Cameron in 2015.
The opposition Labour Party could win 257 seats, up from 232 seats in 2015, YouGov said. Smaller parties, including the Scottish National Party and Northern Irish parties, could win 83 seats, The Times newspaper quoted YouGov as predicting.
If the YouGov model turns out to be accurate, May would be well short of the 326 seats needed to form a government in June, when formal Brexit negotiations are due to begin.
May called the snap election in a bid to strengthen her hand in negotiations on Britain's exit from the European Union, to win more time to deal with the impact of the divorce and to strengthen her grip on the Conservative Party.
But if she does not handsomely beat the 12-seat majority Cameron won in 2015, her electoral gamble will have failed and her authority could be undermined just as she tries to deliver what she has told voters will be a successful Brexit.
Sterling traded half a percent lower against the U.S. dollar after the YouGov data was published. It was trading at $1.2800 early on Wednesday.
For scenarios on the election, please click on:
LANDSLIDE TO LOSING?
When May stunned politicians and financial markets on April 18 with her call for a snap election, opinion polls suggested she could emulate Margaret Thatcher's 1983 majority of 144 seats or even threaten Tony Blair's 1997 Labour majority of 179 seats.
But polls had shown May's rating slipping over the past month and they fell sharply after she set out plans on May 18 to make some elderly people pay a greater share of their care costs, a proposal dubbed the "dementia tax" by opponents.
A total of seven polls carried out since the May 22 Manchester suicide attack have shown May's lead over the Labour Party narrowing, with some suggesting she might not win the landslide predicted just a month ago.
The polls painted a complicated picture of public opinion, with voting intentions being influenced by both the deadly Manchester attack and May's unpopular social care proposals.
In contrast to YouGov's model, other projections suggested May would win soundly. The Electoral Calculus website, which predicts the results based on polls and electoral geography, said May would win 371 seats and Labour 205 seats.
Betting markets give a more than 80-percent probability of May winning an overall majority, though betting markets were wrong ahead of the unexpected Brexit result in the June 23 referendum.
The Times said YouGov acknowledged that its predictions were controversial and allowed for a wide margin of error.
YouGov Chief Executive Stephan Shakespeare told The Times that the model had been tested during the run-up to the EU referendum last year and that it had consistently put the Leave campaign in the lead.
The YouGov research allowed for big variations in the outcome of the election, ranging from as high as 345 seats for the Conservatives, 15 more than their current number, to as low as 274, The Times said.
The model allowed YouGov to assess the intention of every type of voter, from where they live to how they voted on Brexit, their age and social background, in order to weight the results.
Shakespeare said the figures could change dramatically before June 8.
"The data suggests that there is churn on all fronts, with the Conservatives, Labour and the Liberal Democrats likely to both lose and gain seats," he was quoted as saying.
For an interactive graphic on the election, click: http://tmsnrt.rs/2q7tC48
(Writing by Guy Faulconbridge; Editing by Andrew Heavens)