The Liberal Democrats have urged Andy Burnham to stop the "bonfire of patient safety" as part of the government's plans to modernise the NHS.
Healthwatch England (HWE), an independent body which represents the views of patients at their local health and social care providers, is set to be scrapped under the NHS Modernisation Bill.
The Health Service Safety Investigation Bureau (HSSIB), which carries out patient safety investigations, is also being abolished.
The government says its changes will strip back bureaucracy and improve patient safety.
The Lib Dems warned it risks another Stafford Hospital scandal – where years of abuse and neglect led to hundreds of unnecessary patient deaths.
A Healthcare Commission report found the scandal – one of the biggest in NHS history – involved at least 400 more deaths than expected between 2005 and 2008.
Under the government proposals, Healthwatch's responsibilities will be transferred to integrated care boards (ICBs) and local authorities, meaning they would have to respond to feedback about their own services, rather than being challenged by an independent body.
Separately, HSSIB's functions will be transferred to the government-run Care Quality Commission (CQC), which the Lib Dems argue raises concerns there is no guarantee that staff who raise safety concerns are given anonymity.
Other measures include the abolition of NHS England in a bid to centralise the running of health and social care services under the Department of Health and Social Care (DHSC).
The government says this will reduce bureaucracy and allow more resources to be delivered to frontline healthcare services.
NHS England to be axed as role returns to government control
The NHS Modernisation Bill is currently at the report stage and due for a third reading on 7 September.
Liberal Democrat health spokesperson Helen Morgan said this was "a catastrophic step backwards" and described it as the "greatest backward step in patient safety in a generation".
"Andy Burnham must urgently put an end to this, and fix the Health Bill so that modernising the NHS doesn't come at the expense of accountability, transparency or patient safety," Morgan added.
In a speech on NHS reform in January, then-Health Secretary Wes Streeting said "there are many decent people working at Healthwatch who care, but the job of listening to patients can't be farmed out to an arm's length body".