A new organisation designed to make the NHS in England safer must have its independence guaranteed in law, a committee of MPs says.
The Healthcare Safety Investigation Branch (HSIB) is due to begin work this autumn with a budget of £3.6m and will carry out about 30 reviews a year.
The MPs' report also calls for a single public inquiry into historical cases of avoidable harm in the health service.
The government says it has made legal provisions for HSIB's independence.
The cross-party Public Administration and Constitutional Affairs Committee (PACAC) has been examining the work of the new body, which will sit within NHS Improvement (a body which oversees NHS Trusts) - a move which has been controversial.
A chief investigator will be appointed soon.
The aim is to model the "no blame" approach taken to investigating air accidents.
The hope is this will enable NHS staff, patients and their families to raise awareness about serious risks to safe care, and allow hospitals and other providers to learn from mistakes.
But patients' organisations have warned that this "safe space" should not be prioritised above guaranteeing openness and honesty with patients.
And the PACAC report warns the organisation will fail without new legislation.
'Safe space' under fire
Committee chairman Bernard Jenkin MP said: "We have consistently called for primary legislation to make HSIB fully independent, and to create a credible 'safe space' which will enable the NHS to properly learn from past mistakes.
"The secretary of state's decision to set HSIB up as an NHS quango as a permanent response to our recommendations was both disappointing and would be unacceptable.
"But the prospect of a secure legislative base will enable HSIB to emulate the successful air, marine and rail investigation branches.
"Were the present non-statutory arrangement to be regarded as permanent it would be an intolerable compromise."
Ministers have made no commitment to legislation, but say they will review how HSIB is working after two years.
Scandals at Mid Staffs and Southern Health Trust have highlighted failings in how the NHS responds to patients' complaints and deaths.
The MPs say the HSIB should be "an exemplar of high quality clinical investigations", with local NHS providers still carrying out the bulk of inquiries.
But the report warns there will be limits to what it can achieve, within its budget and remit.
'Wide-reaching' public inquiry
The MPs also recommend that a single public inquiry selects and reviews historical cases of failings in the health service.
They say: "This should be seen in the context of other wide-reaching inquiries in recent years, such as the public inquiry into historic child sexual abuse, the inquiry into the Hillsborough disaster, and the Savile inquiry into the events of Bloody Sunday.
"The purpose of this single public inquiry would be to provide closure to those affected by patient safety incidents, which cannot otherwise be obtained."
Patient safety campaigners at the charity Action against Medical Accidents (AvMA) questioned whether the no-blame approach was the right one.
Its chief executive Peter Walsh said: "We believe that both PACAC and the government are wrong to prioritise the creation of a so-called 'safe space' for health professionals above guaranteeing openness and honesty with patients or their families about their own treatment.
"This would undermine public confidence in HSIB and run against the principle of the newly created duty of candour.
"Of course we do want to see protection of staff who do the right thing, but most health professionals would agree that denying patients access to the truth is no way to do that."
A Department of Health spokeswoman said: "The new independent HSIB will carry out investigations and share its findings to improve patient safety.
"We agree that the independence of the branch will be essential to its success, which is why we made explicit legal provisions for its independence when we set it up."