Newly-elected MP George Galloway has warned Sir Keir Starmer that areas including the West Midlands could swing their support away from Labour.
It comes after the Workers Party of Britain candidate won 39.7% of the votes in a by-election in Rochdale.
The vote saw the major parties fail to win the seat, with a local business owner finishing second in the poll behind Mr Galloway.
He has since confirmed that his party will put forward candidates at the General Election – and has predicted that voters in areas including the Midlands would continue to lose faith with Labour.
“Beginning here in the north west, in the West Midlands and in London, from Ilford to Bethnal Green and Bow, Labour is on notice that they have lost the confidence of millions of their voters who loyally and traditionally voted for them, generation after generation,” he said.
The Rochdale by-election was called following the death of Labour MP Tony Lloyd who had served as an MP for more than three decades.
Galloway, leader of the Workers Party of Britain, focused his attention during his campaign on supporting Gaza and criticising Sir Keir Starmer for refusing to back a ceasefire in the Middle East.
However, the Labour leader dismissed the significance of the result after the party withdrew support for candidate Azhar Ali over comments regarding Israel.
“Galloway only won because Labour didn’t stand a candidate,” Sir Keir said. “I regret that we had to withdraw our candidate and apologise to voters in Rochdale.
“But I took that decision. It was the right decision.”