One of the Birmingham Six has challenged the IRA members behind the 1974 pub bombing atrocity in the city to "come clean" and own up to their part in the massacre.

Paddy Hill will launch an online petition this week aimed at pressurising the government into a new public inquiry into the bombings, which killed 21 people and injured 182 others.

In a speech this weekend to commemorate the 1972 Bloody Sunday shootings in Derry, Hill called for the remaining IRA activists to tell the truth about what happened in Birmingham.

He also said he hoped any fresh investigation would highlight the role of an alleged informer who, Hill claims, told West Midlands police that he and the five other people wrongly convicted of the murders were not involved in the bombings .

Hill said: "From what we have learned we now know that there was an informer in the IRA unit that bombed Birmingham. At the time of our arrests he told the police that we were not even in the IRA and knew absolutely nothing about the bombs. Any new inquiry has to look at the role of that informer in this scandal.

"As for those behind the bombs, there are, to my knowledge, three of them still alive, walking the streets as free men. I don't believe they would do a single day in jail, due to the amnesty given under the Good Friday agreement. But I do feel they should come clean and tell any public inquiry exactly what they did 40 years ago."

Hill said any new inquiry should also explore the role the then Labour government played in putting him and five other innocent men behind bars.

"There is a 75-year public interest immunity certificate on our case, which prevents the full truth coming out. Any public inquiry should be allowed access to all that material and overturn that bar on the full facts. Because even at the time of our arrests, we were told by police officers that they didn't care if we did it or not – that people right at the top needed convictions. That has to be looked at in any inquiry."

Hill said he was "astonished" that the families and loved ones of those killed and maimed in the 1974 IRA bombings failed to get the necessary numbers to sign their own online petition. "That's why I am lending my help with my own online petition, which needs 100,000 plus people to sign."

Paddy Hill and five other Irishmen – Hugh Callaghan, Billy Power, Johnny Walker, Richard McIlkenny and Gerry Hunter – spent 16 years in prison. In 1991 the court of appeal quashed their convictions for the Birmingham bombings.