Storm clouds are gathering over the Health and Social Care Bill, the Government's proposed shake-up of the NHS in England, as the House of Lords prepares to debate it next Tuesday.

Hard on the heels of the letter from 400 public health experts who wrote to peers to warn that the Bill would cause "irreparable harm", comes another letter to noble Lords from two eminent crossbench peers, urging that key sections should not be debated in detail by the full House of Lords, but instead scrutinised by a special select committee - it would be a "two-speed appraisal" of the much delayed and rewritten Bill.

Lord Owen - former leader of the SDP, former surgeon and a Labour health minister in the mid-1970s - and the leading constitutional academic Lord Hennessey believe one section of the Bill has serious constitutional implications - and deserves much more detailed consideration than it could be given in a regular Committee Stage debate in the House of Lords.

One target for this more detailed treatment are the nine Clauses setting out the duties of the Secretary of State. The Lords Constitution Committee, which examined the constitutional implications, suggested that Peers would need to consider "whether these changes pose an undue risk either that individual ministerial responsibility to Parliament will be diluted or that legal accountability to the Courts will be fragmented".

They also want a committee to look at Part 3 of the Bill relating to the NHS competition quango, Monitor. They hope a select committee would flesh out its precise purpose and operational methods and consider the implications for EU competition law. And they want a longer look at the role for public involvement and local government, in the running of the NHS.

All of these, they argue, are areas where the select committee format, which allows witnesses to give evidence, would provide a much stiffer examination of the Bill's proposal. They even want any select committee to hire a parliamentary draughtsman "to help with the wording and the presentation of the many amendments that this very large Bill is likely to attract".

This is rather more than a procedural wrangle about how to handle the Bill. It has already been much delayed and amended in the Commons - and now, it seems, influential peers are gearing up to re-write large sections of it again.

Viewed against a background of Labour opposition, strong doubts on the Lib Dem benches, the emergence of strong crossbench opposition suggests that Health Secretary Andrew Lansley has a lot more compromising to do, to get his flagship Bill into law.

Those who remember the bitter recriminations around the demise of the SDP in the late 1980s can only marvel at the emergence of a cause which appears to have united Dr David Owen and his former Gang of Four partner Shirley Williams (who promised the Lib Dem conference, a couple of weeks ago, that the Lords will scrutinise the Bill "line by line").

It is a clear sign that a key piece of the Government's programme faces further erosion.

***** UPDATE 1700 BST: Baroness Thornton, who leads for Labour on health in the Lords, has now announced they will be supporting the Owen-Hennessey amendment. That means it has an excellent chance of coming to pass, since Lady Williams and other Lib Dem dissidents have been complaining that the Bill has not had the scrutiny it deserved in the Commons. Labour Peers, plus some Crossbenchers, plus some rebel Lib Dems could well put together a majority, especially if bolstered with Conservative Peers from the Constitution Committee - whose worries were quoted by the wily Lords Owen and Hennessey.

"Lord Owen's amendment means an important and complex part of the Health Bill will now be subject to the close scrutiny it failed to get in the Commons," Lady Thornton said. "A committee of experts will be able to take evidence - something normal procedure in the Lords does not allow. They will also be able to recommend whether these parts of the Bill make sense and how they might be improved."

"Parts of the Bill, such as the Failure Regime - dealing with closure or sale of services and hospitals - were introduced so late in the Commons that they have not been looked at all."