Eight-day old Amitai Moshe went into cardiac arrest 15 minutes after the operation was performed and died eight days later. The Metropolitan Police's serious crime directorate is leading an investigation into his death.

A police spokesman said he was taken to the Royal Free Hospital by the Hatzola ambulance service at about 8.25am on February 1. He was later transferred to University College Hospital, where he died on February 9.

An initial postmortem examination found the baby suffered a cardiac arrest and oxygen starvation to the brain. The cause of death has not been confirmed and police are treating the death as unexplained, the spokesman said.

He added: "Inquiries are being carried out by officers from the specialist crime directorate to establish the circumstances that led to the child becoming ill. His death is being treated as unexplained at this early stage."

The Bris Milah ceremony was carried out by a registered practitioner of circumcision, known as a Mohel, at Golders Green United Synagogue.

The rabbi of the synagogue, Harvey Belovsky, said: "It goes without saying that our main task has been to provide all the support and sensitivity possible to the family at this time. As a rabbi, it is reassuring for me to see that they feel so helped by the members of our congregation, who have rallied to its side throughout this extremely trying and difficult period."

A spokesman for the Board of Deputies of British Jews said: "No causal connection has been established between the circumcision and Amitai's death. It was carried out by a mohel who is a registered member of the Initiation Society, which has been regulating and training them for over 200 years. Over 2,000 circumcisions of baby boys are carried out in Britain every year. It is a very regulated practice in terms of medical training. This was an unfortunate juxtaposition of two events."

The Initiation Society recorded 2,000 circumcisions last year - but many more are carried out. One in 10 teenagers is circumcised. Circumcision is common among Muslim communities, and in some areas it is available on the NHS. Serious complications are very rare.

Some doctors argue there is no strong medical evidence to support routine circumcision, particularly as it is known the baby experiences pain, though others recognise the religious and cultural importance of the practice. Su-Anna Boddy, a consultant paediatric urologist at St George's hospital in London, said it was likely local anaesthetic would have been used. "To my opinion and knowledge, the Jewish faith needs this to be done and have an excellent way of doing it."

David Smith, general manager of Norm UK, a registered charity which campaigns for circumcision to only be performed for adults who can give consent, said: "Medical opinion is that this is a damaging procedure. We feel only people who can chose should be given circumcisions."