Sharon Graham accused David Lammy of launching a "direct attack" on British Airways workers.
“This is not a moment for posturing and standing on picket lines,” the shadow foreign secretary said.
The prime minister is planning a "summer sausage offensive" to shore up support among MPs, according to reports.
David Lammy: “We want to win outright and we will be fighting for every single vote in every single constituency."
The under fire prime minister has predicted he will lead Britain into the 2030s, sparking ridicule from some of his own MPs.
He risked further angering his Tory critics by claiming he was already planning for a third term in office.
The prime minister said voters want him to get on with the job.
The prime minister is out of the country as political and economic turmoil sweeps the UK.
Prime minister says he expects people will "continue to beat me up" following by-election losses.
The prime minister said: “I have always believed in a woman’s right to choose and I stick to that view."