Gordon Brown has dismissed the British Airways (BA) cabin crew strike as "deplorable", despite its backers being one of Labour's main funders.
The Unite union is resisting government demands that it backs down as holidaymakers face travel disruption in the run-up to Easter.
"It's the wrong time, it's unjustified, it's deplorable," the prime minister said on BBC Radio 4's Woman's Hour programme.
"We should not have a strike. It's not in the national interest."
BA will be hit by two stages of industrial action, from March 20th to 22nd and from March 27th to March 30th, in a dispute over staffing changes on long-haul flights.
No 10 was initially reluctant to confirm a report in the Guardian newspaper that the prime minister has sought to intervene by calling Unite's joint general secretary Tony Woodley.
This may have been because Unite, which has donated £11 million to Labour in the last three years, remains defiant in the face of government pleas.
"Lord Adonis appears badly informed," a spokesman for the union said yesterday.
"We all want to avoid strike action and Unite is always ready to negotiate. Unite was preparing to put BA's offer to our members. Had they accepted it, there would be no strikes."
Mr Brown pressed for negotiations, which he claimed nearly succeeded in reaching a deal last week, to continue.
"I hope that things that are in hand today will lead to the sides talking and finding some resolution," he added.
"It is worthy of effort to try to prevent it."
His comments followed transport secretary Andrew Adonis' condemnation of Unite yesterday.
"The impact will not only be deeply damaging on passengers, it will threaten the very existence of British Airways," transport secretary Lord Adonis - who is a non-political member of the government - said on BBC1's The Andrew Marr Show yesterday.