Three members of an armed teenage gang who killed a woman as she cradled a baby at a christening party were illegal immigrants with a string of convictions.

Diamond Babamuboni, 17, and his brother Timy, 15, were under supervision orders when Zainab Kalokoh was shot in the head. Jude Odigie, 16, had been given a conditional discharge. They were convicted of manslaughter at the Old Bailey yesterday.

A 17-year-old, who cannot be named for legal reasons, was convicted of murder. All four face long prison sentences.

The conviction of the Babamuboni brothers and Odigie, all from Nigeria, raises questions about the Home Office’s immigration service and the criteria for deportation.

The trio could be sent home after serving their sentences, but John Reid, the Home Secretary, is certain to be asked why this had not already happened.

The youths were members of a gang of armed, masked raiders who burst into the party on the Wood Dene Estate in Peckham, South London, on August 27 last year to rob guests.

Mrs Kalokoh, 33, who had fled war-torn Sierra Leone, was among 100 guests at the party. She had taken charge of her niece, six-month old Adama Yalie, whose christening the guests were celebrating, while her mother went to the toilet.

During the trial Brian Altman, for the prosecution, described Mrs Kalokoh’s last moments. He said that the people at the party had been talking and dancing when the gang arrived. They were “heavily armed, with loaded guns and intent on extreme violence, and extreme violence is what they did that night”.

Mrs Kalokoh was shot in the head with a 9mm handgun. She collapsed to the floor with the baby still in her arms. Although the baby was covered in Mrs Kalokoh’s blood she was “mercifully unharmed”, but Mrs Kalokoh died quickly.

As her life ebbed away, the raiders stripped other guests of handbags, purses, mobile phones, food and wine at gunpoint, gathering their loot in black bin liners.

The Babamuboni brothers came to Britain with their mother after her husband died. She was refused refugee status and is still facing deportation after unsuccessfully lobbying Harriet Harman, their MP, to help them to stay in Britain.

Miss Harman, now Minister for Justice, insisted that she was only “checking the progress” of the case when she wrote to the Home Office and local housing department.

Diamond had a string of previous offences beginning when he was 12.

In 2001 he was given a conditional discharge for driving without a licence. Nine months later he was back in court for burglary and in the same year received a six-month detention and training order for robbery.

This was followed by a further four-month detention order in 2003 for possessing a knife. In 2004 there were court appearances for shoplifting, attempted robbery, robbery, theft and breach of an ASBO. For the last two offences he was put under supervision for 12 months.

His brother’s first conviction was for robbery when he was 10. In 2001 he pleaded guilty to using a knife to threaten a girl and rob her of her mobile phone. In 2002 Timy Babamuboni pleaded guilty to theft, and in 2003 was given a supervision order. It was followed in 2004 with another two-year supervision order for assault with intent to resist arrest.

Jude Odigie, 16, was an overstayer who had been refused leave to remain in Britain as a dependant of his mother. He had two convictions last year for disorderly behaviour and failing to surrender to custody, and had been given a conditional discharge.

In a statement the Home Office offered its sympathy to Mrs Kalokoh’s family but did not comment directly on her killers. “We want to make clear that we will not accommodate those that abuse our hospitality and sanctuary by becoming involved in crime.”