New analysis by pro-EU campaign group Best for Britain suggests that as few as 40,704 tactical votes across just 36 marginal seats would be enough to prevent a Conservative majority.
It says that although the Tory majority has halved in the space of a fortnight, without tactical voting Boris Johnson is still on course to win 345 seats and a majority in Parliament.
The West Yorkshire seat of Dewsbury, where Labour's Paula Sherriff defends a majority of 3,321, is the constituency where Best for Britain says the fewest tactical votes could stop the Conservatives winning.
The group's seat-by-seat analysis suggests it would take just 82 Green and Lib Dem voters backing Labour to prevent the Conservative Party winning the seat. Dewsbury is currently on course to be won by the Conservatives on December 12.
Number five in the list of 36 target seats is Colne Valley, where 256 tactical votes would be needed, while Wakefield was at number 12 with 585 tactical votes required.
Best for Britain says 1,605 tactical votes would be needed to win Tory-held marginal seat Pudsey for Labour, placing it 27th in the list.
Best for Britain, which has its own tactical voting website to prevent a Tory majority, will present the findings of its polling at a press conference on Monday.
CEO Naomi Smith said: "The Tories will be quaking in their boots today. Just a fortnight ago Boris Johnson was on course to win a landslide, but weeks of telling lies and ducking interviews have caught up with him.
"Now he faces the very real possibility of remainers in just thirty-six marginal seats using tactical voting to cannibalise what's left of his expected majority.
"This election is on a knife-edge, and if enough remainers hold their nose and vote for the candidate with the best chance of stopping the Tories in their seat we're heading for a hung Parliament and a final say referendum."
The Yorkshire Post understands that Conservative campaign strategists are planning a final assault on Yorkshire as they go in search of a majority on Thursday.
It will see the Government’s most senior cabinet members including Sajid Javid, Michael Gove and Priti Patel deployed as part of a series of daily ministerial visits through to polling day.
The party needs nine more seats to win a working majority government and is targeting Scunthorpe, Rother Valley, Bradford South, Halifax, and Don Valley as five that will get them over the line.
Separately, new research shows more than half of Labour Remain voters would "vote tactically" to stop a Tory victory where they live.
Polling of more than 10,000 people carried out last week by Populus Data Solutions (PDS) shows Remain-backing voters could switch sides to prevent a Conservative MP being elected in their constituency.
Independent crossbench peer Lord Cooper of Windrush, the chair of Populus and a former head of strategy for the Conservative Party ahead of their 2015 election victory, said this presents a problem for Boris Johnson.
He added: "With less than a week to go before polling day, two plain truths emerge from analysis of the polling.
"Firstly, there is absolutely no chance whatsoever of this election producing a majority in the House of Commons either for Jeremy Corbyn to be Prime Minister, or to pass the irresponsible and extreme elements of his agenda that worry so many mainstream voters.
"Secondly, as shown powerfully in this big new poll, if those who say they are willing in principle to vote tactically to avert hard Brexit actually do so, there is still a real chance of preventing it."