The House of Lords will debate the government's controversial health reforms on Tuesday, as medical organisations unite in opposition to the changes.
The Chairman of the Academy of Medical Royal Colleges, Professor Sir Neil Douglas, has expressed serious concern about the NHS reforms, saying the Bill could "damage patient care".
And the BMA has written to every peer in the Lords outlining their concerns about the Bill.
London university academics have also written to medical journal the Lancet saying are the reforms "fundamentally flawed".
Lords could demand changes to delay the bill and send controversial measures in it to be examined by committee - meaning a serious delay in the timetable for the changes. At least 100 peers are expected to speak before the vote, and former health secretary Lord Owen has tabled an amendment which would send part of the Bill to a committee.
Labour's new shadow health secretary offered to work with the government if they halted the reforms on Sunday.
In a statement Andy Burnham said: "It is plain for all to see that Andrew Lansley has failed to build a consensus in the country behind his reforms to the NHS.
"He has created confusion and uncertainty when the NHS desperately needs clarity and stability. He has put the NHS in the danger zone. He must stop digging in, listen to staff and urgently change
On Monday night Burnham told the House of Lords it was "time to put the NHS first" - and Labour's health team have said the issue "might be the most important week in the history of the NHS since it was created by Labour".
Last week Andrew Lansley said he was willing to make one more change in the NHS bill - to shore up legislation which will make the health secretary legally responsible for the NHS.
But he told delegates at the Conservative party conference that the "fundamental principles" of the coalition government's NHS reforms had not changed despite the pause in the legislation.