The British jihadi linked to the latest murderous Islamic State (IS) propaganda video was arrested six times before fleeing the UK for Syria, it has been revealed.
It was also disclosed that Abu Rumaysah was sent a letter asking why he had not surrendered his passport more than a month after he was supposed to have handed his travel documents to authorities.
Questions are mounting over lapses that allowed the former bouncy castle salesman to leave Britain despite being on bail following claims that he was suspected of being a masked militant at the centre of an IS film showing the murder of five men accused by the terror group of spying for the UK.
Rumaysah, who was born a Hindu and called Siddhartha Dhar, was arrested in September 2014 - when he was 31 - along with eight other men as part of an investigation into alleged support of the banned extremist group al-Muhajiroun.
However, he left Britain with his family the day after being freed on bail, travelling to Paris and then Syria where he posted a picture of himself on social media, holding his baby son in one hand and an AK-47 assault rifle in the other.
Taunting authorities, he wrote on Twitter: "What a shoddy security system Britain must have to allow me to breeze through Europe to (IS)."
Shadow Home Secretary Andy Burnham said there had been a "serious lapse in security".
He told the Commons: "Whether or not he is the person in the video doesn't matter. The system has failed because it allowed him to abscond to Syria.
"He was well-known to the authorities, having been arrested six times on terror-related offences before being placed on police bail in 2014 and requested to surrender his passport. It was when he failed to comply with those bail conditions that it's emerged he had absconded."
A letter was sent to him after his bail on September 26 2014, reminding him of the need to surrender his passport by October 3, but it was not sent until more than five weeks later, on November 7.
Quoting from the letter, Mr Burnham read: "It has come to our notice that condition number three has not been complied with, or so our records suggest.
"Are there any changes to your circumstances that the police need to be aware of? Could you please contact the police on the telephone number listed above as a matter of urgency."
Mr Burnham said: "Does this in any way sound adequate to the seriousness of the charges concerned? It is clear he had left the country long before this letter was sent."
In a statement earlier, Home Secretary Theresa May said she could not comment on the alleged identities of the man or a young boy - also reported to be British - in the ten-minute film posted online.
She said bail decisions and conditions were an "operational matter" for police.
Mrs May said that since April exit checks have been in place on air, sea and rail services while 24 passports were removed from people intending to travel for terrorism-related activity in 2014.
New powers to temporarily seize the passports of those intending to leave the UK in connection with terrorism-related activity have been used on more than 20 occasions, Mrs May said.
The Government's independent reviewer of anti-terror legislation, David Anderson QC, told the BBC: "I stress that I can't confirm that this is Siddhartha Dhar or not, but if we assume for a moment that it is, what is unusual about this case is that he managed to make that journey despite being subject to the criminal justice process, despite having been arrested and placed under police bail."
Mr Anderson played down suggestions that security agencies should have been monitoring Dhar, saying: "There are thousands of people who are subjects of interest. I don't know how high up the list this person was - I suspect not very near the top."
London Mayor Boris Johnson said he would be speaking to Scotland Yard about the episode.
Keith Vaz, chairman of the Home Affairs Select Committee, said "serious questions" should be asked, adding: "Someone on bail who is known to the relevant authorities for counter-terrorism issues should never be allowed to leave the UK."
Security services have completed an "initial assessment" of the identity of the masked individual with a British accent who appears in the film.
Reports claim that a young child seen at the end of the video dressed in military-style clothes, was Isa Dare, the son of jihadi bride Grace "Khadijah" Dare, from Lewisham, south-east London.