Labour's national campaign coordinator Pat McFadden says the Blackpool South by-election was "the one where Rishi Sunak and the Conservative government were directly on the ballot paper."
"We got a stunning victory with a 26% swing, and the Tories struggling even to come in to second place," he said.
The senior Labour frontbencher rejected Tory arguments that the circumstances of the former Tory MP resigning amid a lobbying scandal affected the vote.