Ministers are admitting that Andrew Lansley's controversial health reforms could be killed off for good if what promises to be a knife-edge vote in the Lords goes against the government this week.

The cabinet and the Department of Health have been put on full alert to try to prevent a "peers' revolt" on Wednesday that would lead to key parts of the health and social care bill being referred to a special committee – a process that could take months.

One senior figure in the government said there was now a chance that the bill could be so badly stalled in the committee that it would fail to get royal assent by next spring – meaning it would fall.

The loss of a major piece of legislation would be a huge blow to David Cameron's government, and ministers are determined to do what they can to influence the peers. But peers said privately that they would not be bullied into backing a bill with which they disagreed.

More than 80 peers are down to speak in the second reading debate, forcing business managers to extend the time assigned to it from one day to two: Tuesday and Wednesday.

Ministers are expected to offer further concessions to opponents of the bill – particularly to ease concerns that the responsibility of the secretary of state to provide free, comprehensive healthcare will be watered down – in order to head off a rebellion.

Labour, which will vote for the amendment calling for the referral of key sections to a committee, tabled by former foreign secretary David Owen and the constitutionalist Peter Hennessy, says around 80 crossbench peers and Liberal Democrats will also need to back it to win the day.

In a letter to Lansley, Andy Burnham, the new shadow health secretary, calls on him to withdraw the bill. In return Burnham says Labour would offer its support "in establishing true clinician-led commissioning in every locality in England". Burnham adds: "This could be achieved by asking GPs and other clinicians to establish a representative commissioning team in each PCT area working within the existing statutory framework and preserving co-terminosity with local government."

Last night, the Observer obtained an internal memo, reflecting panic in Whitehall, sent by the chief executive of the NHS, Sir David Nicholson, to senior civil servants, spelling out the government's line that more delays to the bill would be hugely damaging.

Nicholson says that if plans to save £20bn are to be achieved through efficiency savings by 2015, with the money reinvested in the service, then the reforms need to be implemented as soon as possible.

"The scale of this challenge is unprecedented in the history of the NHS. Modernisation offers many opportunities to support the NHS in delivering this quality and productivity challenge," he says. "Greater patient involvement, stronger clinical leadership and reduced overheads can all make important contributions. But the NHS needs clarity and consistency, particularly about the timetable for modernisation."

He adds that "further delay risks creating a leadership vacuum at local level" that could damage patient care.

In a letter to fellow crossbench peers last week Owen and Hennessey stressed the duty of the Lords to scrutinise and amend where necessary: "We believe there has been insufficient scrutiny of parts of the massive health and social care bill and while we accept it is not the role of the House of Lords to challenge the legislation in its entirety which has been given a third reading in the House of Commons, we do believe it imperative the House of Lords provide a mechanism for far greater in-depth consideration of a number of parts of this bill which cover duties and constitutional issues."

Owen and Hennessey believe the special committee should consider whether the bill poses an "undue risk either that individual ministerial responsibility to parliament will be diluted or that legal accountability to the courts will be fragmented".

Other crossbenchers were keeping their powder dry last night. Baroness (Molly) Meacher said she was undecided: "The financial problems of the NHS arise from politicians' failure to lead from the front and close redundant acute hospitals, A&E departments and wards. The top-down reorganisation is irrelevant and costly. It will not begin to touch the financial pressures."

A senior government minister said the dilemma for the coalition was how far it should try to influence the peers before the vote. "The worry is that we might have the reverse effect to that we intend if we do that," said a source.