The LBLA was set up to allow London borough legal teams to share expertise and to lower costs by using a single panel.
The initiative will be launched officially on 15 October, but it is understood that Ashfords, Bevan Brittan, Sharpe Pritchard and TLT have won places on the 16-firm roster.
The LBLA began tendering for the panel places in March this year. Since the process began the LBLA has gained a new member, with Kensington and Chelsea joining Camden, Hammersmith and Fulham, Harrow, Hillingdon and Hounslow in the alliance.
The initiative is expected to save its members a combined total of around £1.5m a year.
Harrow head of legal Hugh Peart said: “We have a duty to get the best value we can for council-tax payers.
“I’m confident that the London Boroughs’ Legal Alliance will do just that, and I’m hoping that other London councils will now decide to come on board.”