There are concerns that the bill does not specify a cap on how much private work foundation trusts can do

Andrew Lansley will have to make further concessions on the Government's controversial NHS reform bill when it gets debated in the House of Lords, the Liberal Democrat peer Shirley Williams said yesterday.

Baroness Williams claimed her party had already won significant changes to the bill to ensure greater local accountability. But in a sign of continuing coalition tensions over the reforms, she added: "It's not perfect and we've still got some way to go."

Paul Burstow, the Liberal Democrat Health minister, said the Government was still open to change.

"We didn't stop listening when the listening exercise ended," he told the conference in a question-and-answer session on health.

"We haven't stopped reflecting on the concerns that colleagues have. We haven't stopped making changes where we think they are necessary.

"The House of Lords revises, it improves, it challenges, and that is what it will do with the Health and Social Care Bill. That is exactly what it should do."

Among the areas in which Liberal Democrat campaigners are looking for further movement is enshrining the Health Secretary's ultimate responsibility for the NHS in the bill and reinstating a cap on how much income hospitals can get from treating private patients.

Baroness Williams is concerned that the bill, as currently drafted, could allow Mr Lansley or his successors to claim that they do not have the power to intervene and keep hospitals and other services open – against the wishes of local NHS managers.

There are also concerns that the bill does not specify a cap on how much private work foundation trusts can do.

Health campaigners fear that as the NHS tries to make efficiency saving of £20bn over the next four years, hospitals will try to increase their private revenue at the expense of NHS patients.