A “fundamental reform” is needed to give bereaved families “a level playing field” in the justice system, Hillsborough campaigners have said.
Margaret Aspinall, whose son died in the disaster, and mayors Andy Burnham and Steve Rotheram have called on party leaders to back a “Hillsborough Law”.
It comes after match commander David Duckenfield was found not guilty of gross negligence manslaughter.
The trio described the current system as “deeply flawed” and “insensitive”.
The families of the victims and survivors have been campaigning for more than 30 years to discover how and why 96 Liverpool football fans died in the UK’s worst sporting disaster on 15 April 1989.
In April 2016, jurors at the Hillsborough inquests concluded the 96 who died in the disaster were unlawfully killed.
In June 2017 the Crown Prosecution Service announced Mr Duckenfield would be charged with manslaughter by gross negligence of 95 people.
Under the law at the time, there could be no prosecution for the death of the 96th victim, Tony Bland, because he died more than a year and a day after his injuries were caused.
But on Thursday Mr Duckenfield was cleared of the charge against him following a retrial at Preston Crown Court.
“The families find themselves in the position where 96 of their loved ones have been found to have been unlawfully killed and nobody has been held accountable for that,” the campaigners said in the letter.
“As we have seen, the different legal proceedings – public inquiry, prosecutions and inquests – are not connected, so each successive process can seek to undermine previous findings, however clear and exhaustively reached,” they said.
“This puts families through torture each and every time.”
Ms Aspinall, Greater Manchester Mayor Mr Burnham and Liverpool Metro Mayor Mr Rotheram have written to the three main political party leaders in England, urging them to introduce a “Hillsborough Law” if they become Prime Minister.
This includes the reintroduction of the Public Authority (Accountability) Bill which would place a duty of candour on all public officials and require parity of legal funding for bereaved families and public bodies.
Mr Burnham has previously outlined the proposed “Hillsborough Law” in the Commons but this fell at the 2017 election.
If a change in the law is agreed, it will “be of some comfort to the Hillsborough families for it to be recognised by the country that the failure is not theirs, but of an overly hierarchical and adversarial system which is deeply flawed and in need of fundamental reform,” the campaigners said.