Campaigners are turning the screws on Jeremy Hunt ahead of a Lords vote which some say would privatise the NHS.
Voluntary groups have accused the health secretary of driving them out of the health service altogether by allowing companies to bid for NHS services.
The news was seized on by shadow health secretary Andy Burnham, who said campaigners needed to lend the opposition support to maximise the chances of the Lords voting down the legislation.
"Need your help. 6 days to Lords vote on NHS privatisation," he wrote on Twitter this morning.
"Coalition doesn't have your permission to put NHS up for sale. Let's remind them."
A coalition of charities, including Marie Curie Cancer Care, Sue Ryder and Help for Hospices, wrote a briefing document seen by the Financial Times highlighting the effect the new changes would have on their operations.
"If the voluntary sector is forced out of the market then this will have a negative impact on patients and families and also the communities in which they live," they said.
It may appear as if the legislative programme for the NHS reform has been going on for years, but the original plans held very little detail about precisely how services would be tendered out.
In effect, the original debate over NHS reforms under Andrew Lansley - which were so controversial they had to be paused while the prime minister tried to save them – was conducted with precious little detail about how the plans would operate in practice.
The detail of how tendering would take place was revealed when the government published Section 75 earlier this year. This secondary legislation appeared to force GP consortia to offer services to private firms and deny them the chance to stick with current providers.
Campaigners said that will leave the public sector with all the expensive, complex services while the private sector took the cheaper, easier alternatives – effectively gutting the NHS and privatising the majority of services.
An initial piece of legislation to that effect was withdrawn following sustained protest, but campaigners say the reworded text provides few assurances.
The new legislation stipulates that all services must be offered to private firms unless GPs are sure they can only be provided by the current supplier. Experts warn that standard of proof would be very hard to provide.
A Lords committee recently concluded the new wording did little to alleviate concerns about the original legislation.