33-year-old Aytach Lisani of Brentwood has pleaded guilty to the murder of Christopher Foster on the first day of his trial at the Old Bailey.
Mr Foster was stabbed in Borough High Street on Friday 29 November 2013.
Mr Lisani will be sentenced in due course. The trial continues for another seven people.
Ali Lisani, Mohammed Aalamani, Ricky Huggins, Billy Dove and Rocky Djelal are all being tried on murder charges.
A further two people – Joanna Stevens and Michael Gore – are charged with perverting the course of justice.