Reform UK would win 271 seats – the most of any party – if a general election were to happen tomorrow.
Labour would secure just 178 seats, less than half of the 411 it won last year.
The Tories would plunge to fourth place behind the Liberal Democrats, with just 46 Conservative MPs projected.
The Liberal Democrats, meanwhile, would gain nine extra seats to build a Commons caucus of 81 MPs, while the SNP would once again be the largest party in Scotland.
Both the Greens and Plaid Cymru would gain three seats each to both hold seven slots in parliament.
It would mean a coalition government would be needed, as no one party would have a majority.
This is according to a survey of 11,500 people, which was first published by Sky News.
The report released by YouGov said: “That a clear majority would now vote for someone other than the two established main parties of British politics is a striking marker of just how far the fragmentation of the voting public has gone over the past decade.”
It added: “According to our data and methods, 26% of voters would opt for Reform UK, 23% for Labour, 18% for the Conservatives, 15% the Liberal Democrats, 11% the Greens, 3% the SNP, 1% Plaid, and 2% for other parties and independent candidates.”
According to YouGov, Reform came out top of the polls in 99% of their simulations, with the rest having Labour at the top.