The coalition's reorganisation of the NHS risks diluting the government's "constitutional responsibilities" to the health service, an influential Lords committee has warned.

The health secretary currently has to a legal duty provide key NHS services, such as hospital accommodation, ambulances, maternity and nursing. The NHS bill going through parliament envisages that the health secretary would only have to monitor their provision and intervene in the case of failure. The government would not be legally and constitutionally responsible.

The committee examining the constitutional implications of public bills, chaired by Lady Jay, suggests the House of Lords, which will debate the changes later this month, ought to "carefully 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."

Ministers have promised to amend the bill to ensure the secretary of state remains "responsible and accountable" for the NHS, but the report says "it may well be necessary to amend the bill in order to put this matter beyond legal doubt".

The committee suggests retaining the "relevant wording contained" in the last Labour health bill to ensure full accountability. The peers argue the current bill only makes "a modest contribution towards accountability... the house [of Lords] will wish carefully to consider whether they are sufficient".

The warning comes as peers are about to vote on the shakeup planned by the health secretary, Andrew Lansley, and as more evidence has emerged that patients are losing out in a cash-constrained, more market-responsive system.

Monitor, the NHS regulator, has announced that management in nine foundation trusts in Manchester could be removed for repeatedly failing to meet cancer targets if their performance did not improve.

Another 37 trusts have been warned for failing to ensure that patients are protected "from risk of harm" and failing to provide "treatment and support that meets their needs".

Labour health spokesperson, Liz Kendall, said this showed "David Cameron and Andrew Lansley are in complete denial about what is really happening on the ground. They claim that waiting lists are falling – yet patients, the public and NHS staff know they are rising, including for cancer care."

Labour claims the regulator, the Office of Fair Trading and the competition courts will decide the future of services if the bill becomes law because the secretary of state will have no responsibility for the NHS.

Kendall said: "I hope members of the House of Lords will now act to ensure this vital accountability remains."

The cross-party committee's views will carry weight in the second chamber because members include Lord Irvine, the former lord chancellor, SDP founder Lord Rodgers and Lord Powell, the former Downing Street adviser to Margaret Thatcher. Their views will undoubtedly sway many crossbenchers who hold the key to votes in the Lords.

Evan Harris, the former Liberal Democrat MP and leading health rebel, said the committee was echoing concerns made by Lady Williams, who has been attacking the bill in the upper house. "The government's defence is that the bill says the secretary of state must use their powers under this act to provide services. But there are no powers to do so. Legally and philosophically, they have taken away the secretary of state's ability to do anything."

A Department of Health spokesman said: "While we respect the view of the committee, things have moved on since it did its research and we have already addressed the issue raised... we fundamentally disagree that the bill would dilute the government's responsibilities with regard to the NHS. The secretary of state will continue to be responsible – as now – for promoting a comprehensive health service."