David Cameron is set to accuse Gordon Brown of caving in to "vested interests" in a fresh assault on Labour over its close ties with the trade union behind the British Airways strike.
The Tory leader will suggest the Prime Minister is failing to act and putting thousands of jobs at risk because Unite is "bankrolling" his party.
He will paint it as part of a wider pattern - with bankers, union bosses, wealthy non-taxpayers and the backers of a new runway at Heathrow all getting their way over a "feeble" administration.
And he will promise to "fight" vested interests if the Tories take power at the general election - singling out teaching unions opposed to school reforms as one of his principle targets.
Mr Brown has condemned the strike as "deplorable" and called for it to be halted, but the Tories have seized on the industrial action to highlight Labour's links with Unite.
The union has donated £11 million to the party in recent years and has links to many MPs and election candidates - sparking Tory warnings of a return to 1970s style industrial strife.
In angry clashes with Mr Cameron at Commons question time this week, the Prime Minister told him he should be "ashamed" of using the dispute as a "political football".
But in a speech on Saturday, Mr Cameron will renew the attacks, accusing his rival of having "consistently given in to special interest groups" on several fronts.
Vital public service reforms had been halted because the PM was "too terrified of upsetting the union barons, losing their votes and their money", he will say.
"Powerful lobby groups" secured the green light for a Heathrow runway despite environmental concerns and Mr Brown failed to stand up for taxpayers' interests after the bank bailout. Once again, under Gordon Brown the vested interests triumph and the people lose out," Mr Cameron will say.
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