Relatives of the 21 people killed in the 1974 Birmingham pub bombings have urged police in Britain and Ireland to bring the alleged perpetrators to justice after an inquest ruled the victims had been unlawfully killed.

The plea – directed at police forces in Britain and Ireland – was made at the end of a six-week hearing that resulted in a jury on Friday ruling that a botched warning call in advance of the attacks caused or contributed to the deaths.

Seizing on the evidence of a convicted IRA bomber, who named four men as being responsible for the attacks, relatives demanded that police launch fresh criminal investigations and charge living suspects with what was now officially known as murder.

Julie Hambleton, who has recalled how her “world fell apart” aged 11 when her 18-year-old sister Maxine was killed, said: “West Midlands police have always told us that when they have new evidence they will act on it. Well here you go, and I’m sure there is more to be found when those perpetrators who are are still at large are caught.”

Flanked by other bereaved relatives, Hambleton said “the ball is now in the court” of Dave Thompson, the chief constable of West Midlands police, the Garda Síochána in Ireland and the PSNI in Northern Ireland to pursue suspects who were still living.

She added: “Twenty-one of our loved ones went out one evening to meet friends and family and they never saw the light of day again. Today, the inquest has found they were unlawfully killed by murder. Whilst the police officers of the day can’t be questioned in any great detail, there is no way that the people who this country and the people of our community of Birmingham and families will allow the senior police officers to get off with letting these murderers … to get away with their continued liberty.”

Thompson said: “I would say the West Midlands Police investigation is an active police investigation. If we are able to bring people to justice for the terrible events of 1974 that’s what we will do.”

Sources at the Garda Síochána in Dublin said it would “fully cooperate” with any investigation that is run by the West Midlands Police.

As well as the 21 deaths, more than 200 people were injured when bombs were detonated in two city centre pubs – the Mulberry Bush and the Tavern in the Town – on the evening of 21 November 1974.

Six men who became known as the Birmingham Six were convicted in 1975 of carrying out the bombings, but were acquitted 16 years later.

Fresh inquests into the deaths were ordered in 2016, but were delayed by disputes over whether the hearings should examine who might have been responsible for the bombings. In July last year, the court of appeal decided they should not.

In a verdict read out at Birmingham civil justice centre, the 11-person jury said the deaths were not the result of police errors or omissions and that the victims had been unlawfully killed, a finding directed by the coroner, Sir Peter Thornton QC.

They unanimously ruled that the inadequate warning call by the Provisional IRA cost the stretched police crucial minutes.

It had taken a month in the inquests before the families came close to what they had wanted – the names of the people alleged to have done it. Giving evidence via secure videolink, a convicted IRA bomber known as Witness O said the four men responsible for the bombings were Seamus McLoughlin, Michael Hayes, Mick Murray and James Gavin. They had all been previously named publicly and McLoughlin, Murray and Gavin are dead.

The inquest had heard that a bomb warning using the codeword “Double X” – which was designed to signal it was not a hoax – was received by the Birmingham Post newspaper at 8.11pm. The jury, which was told that police did not try to clear the area or set up a cordon following the call, found that there was no tip-off to the police.

Jurors were told that shortly after 8pm on the evening of 21 November – which was payday for many and a late shopping night – two bombs exploded in the two city centre pubs. There were about 50 people in the Mulberry Bush and 200 in the Tavern in the Town.

The West Midlands had experienced what was described in the inquest as a “serious and sustained” campaign of attacks by the IRA in the preceding 12 months, with 53 bombing incidents in the year leading up to 21 November.

That night there were only 15 police officers in Birmingham city centre because 135 officers from the Digbeth and Steelhouse Lane stations had been deployed to police the funeral procession for the Provisional IRA member James McDade, who had been killed while planting a bomb at the GPO telephone exchange in Coventry the week before.

Facebook Twitter Pinterest The Birmingham Six pictured on 14 March 1991, the day their convictions were quashed. Left to right: John Walker, Paddy Hill, Hugh Callaghan, Chris Mullin MP (who campaigned for their release), Richard McIlkenny, Gerry Hunter and William Power. Photograph: Sean Dempsey/PA

The bombs were detonated between 8.15pm and 8.20pm – first at the Mulberry Bush and then at the Tavern in the Town – but it was not until 9.10pm that extra police officers started arriving, after the Aer Lingus flight carrying McDade’s body left at 8.30pm.

In a police statement from the time of the bombings that was read to the jury, Mary Jones, the wife of David Jones, the licensee of the Mulberry Bush, said she remembered chatting to a group of regulars including Stanley Bodman, John Rowlands, Michael Beasley, James Caddick and John Clifford Jones, all of whom were killed.

Survivors described crawling over bodies to escape the wreckage of the two pubs. Paul Murphy, who was 17 and in the Tavern in the Town drinking with friends, said one of his first thoughts after the blast was that his father was going to kill him for being in a pub. He suffered 33% burns and all of the muscles in his right arm were lacerated.

“The blast had blown most of your clothes off. No hair, no eyebrows, your private parts are covered, because you’re wearing three layers,” he said. “Your socks were blown off to the level of your shoes. You’ve lost that much blood, it’s like when you step in a river, you’re squelching in your own blood.”

A report from a team at the Centre for Blast Injury Studies (CBIS), which was read to the court, heard that the injuries suffered by the 19 victims who died on the night were unsurvivable, “even with current advanced medical treatment”. Thomas Chaytor and James Craig died later in hospital, but the inquest was told it was likely they would have died in any circumstances.

Speaking via videolink from Dublin, the former IRA intelligence chief Kieran Conway denied that the attacks constituted murder and instead described them as an IRA operation gone wrong.

IRA chiefs were told the atrocity was down to the delay in calling in the coded warning because the chosen phone box was out of order, said Conway, a criminal defence solicitor in Ireland who was convicted of handling explosives in Derry in the 1970s. Asked if this could have been a “well-orchestrated and convenient lie” so the bombing team could avoid punishment, he agreed that it could have been.

Giving evidence a few days later, the former MP Chris Mullin, whose investigative journalism was vital in the freeing of the Birmingham Six, refused to give the names of the people he thought had carried out the attack and were still alive. He said they had only spoken to him on condition that he protect their identity.

His evidence produced a furious reaction from the family of Maxine Hambleton, who was killed in the bombing aged 18, with her sister shouting across the court room that Mullin was “a disgrace”. Mullin had to be escorted from the building following his evidence by police and security guards as members of the Hambleton family jostled him and shouted that he had “done nothing for the victims who were slaughtered in cold blood”.