Hammersmith & Fulham, Kensington and Chelsea, and Westminster have run joint provision across children's services, adult social care, and public health since June 2011, but the arrangement is set to come to end on 1 April 2018 when Hammersmith & Fulham Council is formally dropped from the arrangement.
However, Hammersmith & Fulham's director of children's services, Steve Miley, has said that adoption and fostering is the one service area where shared provision will continue.
"Although the formal tri-borough is coming to an end and some services within that did not deliver in the way that we hoped, fostering and adoption is the one area I think where we were all agreed that we wanted to maintain that service," said Miley.
"Even though tri-borough is coming to an end, that shared service will continue."
Miley added that the service was a success because it pooled the three boroughs' registered foster carers, making it easier to find suitable placements for children and reducing the use of independent fostering and adoption agencies by a quarter.
In its January 2016 inspections of the three boroughs, Ofsted judged the service to be "outstanding". Inspectors highlighted its "exceptionally strong" adoption support, "hugely effective planning for permanence" and above-average number of children from ethnic minorities and over-fives being adopted.
The split between the tri-borough authorities followed political tension between the Labour-controlled Hammersmith & Fulham and its Conservative-run partner authorities.
In March, Westminster and Kensington and Chelsea announced plans to ditch Hammersmith & Fulham from the partnership, claiming they had learned that the Labour authority was itself working on plans to withdraw from the arrangement.
In response, Hammersmith & Fulham leader Stephan Cowan said there were "conflicts of interest" within the arrangement and that his borough was not getting a good deal from the partnership.
In the wake of the split, Westminster and Kensington and Chelsea are creating a "bi-borough" arrangement that will pool their children's services, public health and adult social care.
A report for a meeting of Westminster Council's cabinet today (30 October), states that: "This innovative move will enable us to create more unified services, transforming the way we serve families and communities."
The report adds that the move from a tri- to a bi-borough arrangement will put an estimated 18 jobs at risk. However, it adds that the full implications for staff would only become clear nearer Christmas following consultations with staff across the three boroughs. Westminster currently estimates that moving to the new arrangement will cost around £500,000 for itself and Kensington & Chelsea Council.
It had initially been predicted that the cost of the collapse of the arrangement could exceed £1.5m.
Miley revealed that the adoption and fostering service would continue on a tri-borough basis while giving evidence to an education select committee inquiry into fostering.