The multilevel regression and post-stratification (MRP) analysis predicted that Labour would win 431 seats, with Prime Minister Rishi Sunak's Conservatives expected to win 102. It gave Labour a majority of 212 seats.
The pollster said 89 seats were considered "tossups", with five percentage points or fewer between first and second place.
Even in a best case scenario the result for the Conservatives is significantly worse than the party's previous worst election result in 1906, when it won 156 seats, YouGov said.
The Liberal Democrats are set to win 72 seats, and Nigel Farage's Reform UK are forecast to take 3 seats, it added.
