The Conservatives are odds-on to lose both Wakefield and Tiverton & Honiton by-elections on Thursday.
According to the bookies, there is now a 70 per cent chance that the Tories will lose both contests, with Wakefield given a 98 per cent chance of being a Labour gain.
However, Tiverton & Honiton is less certain; the likelihood of the Liberal Democrats winning currently stands at 72 per cent, falling from a high of 90 per cent.
Smarkets projects a Liberal Democrat majority of just over 3,000 votes to be the most likely outcome in that seat.
Commenting on the numbers, Smarkets head of political markets, Matthew Shaddick, said: “The latest prices at Smarkets indicate that a Labour win in Wakefield is close to a certainty.
“Anything else would represent one of the biggest by-election shocks in recent history and would likely pile a huge amount of pressure on Sir Keir Starmer’s leadership.
“The betting on which candidate will finish third has been an unexpectedly lively market – look out for independent Akef Akbar who is now fancied to finish ahead of the Lib Dems, Greens, and others.
“Tiverton and Honiton is harder to call; the Lib Dems are 72 per cent favourites but there has been some money coming in for a Tory hold over the last few days.
“Still, it’s been a very long time since the Liberal Democrats have failed to win a by-election in which they have been competitive and a majority of just over 3,000 votes is the latest market expectation.
“Anything much higher than that may fuel further speculation about Boris Johnson’s prospects of holding onto power for much longer.”