A piece of legislation which campaigners say will privatise the NHS will be voted on by the Lords tonight, in a move which some consider a game-changer for the health service.
Peers will have the opportunity to block the Section 75 regulations which experts say embeds the free market into the NHS.
"Having been caught out trying to privatise the NHS by the front door, ministers are now trying to sneak in the same rules by the back," shadow health secretary Andy Burnham said.
"They are insulting the intelligence of parliament and risking a crisis of trust amongst medical professions.
“The House of Lords has a chance today to stop this relentless march of privatisation to the heart of the NHS."
Campaigners will hold protests outside the Lords today to put pressure on peers over the vote.
The new regulations mandate that any NHS service which can be tendered out to private or voluntary groups must be subject to the bidding process and overseen by competition rules.
Voluntary groups say they will be squeezed out of the tendering process as private firms massively outspend them.
Health experts warn the process could end up dumping expensive complicated procedures on the public wallet while private companies hoover up simple operations. There also worries about the atomisation of services and the effect on joined-up care.
The Section 75 rules are considered by many legal analysts to be the engine of the NHS reforms which caused extraordinary controversy when the coalition came to power.
Since the main body of the legislation was passed, the privatisation of public services has continued apace, specifically with a rise in the number of private ambulances.
"Out there in the NHS, a market is unfolding oblivious to any assurances given by ministers to parliament, and these regulations are a crucial vehicle for its delivery," said Philip Hunt, Labour's health spokesperson in the Lords.
"That is what makes the regulations so dangerous and so different from the guidance issued in 2010."
The British Medical Association joined the Royal College of General Practitioners last week in calling for the regulations to be re-written.
Meanwhile, Marie Curie and other voluntary organisations have warned that mandatory competition will damage patient care.
The Section 75 rules have already been rewritten after they were greeted with outrage by campaigners and faced the very real possibility of defeat in parliament.
But the independent Lords Scrutiny Committee found the new rules are "substantially the same" as the originals.