Westminster and Kensington & Chelsea said they had “reluctantly” taken the decision “in the face of uncertainty caused by.… Hammersmith & Fulham appearing to make alternative in-house plans without any formal engagement with the other two local authority partners about key services”.

The two authorities claimed this was causing anxiety to shared staff and placing potential risks to the joint services for vulnerable people in each borough.

In response the Leader of Hammersmith & Fulham, Cllr Stephen Cowan, said the council had had concerns about the value of Tri-borough and conflicts of interest.

Westminster and Kensington & Chelsea’s decisions will terminate the shared staffing arrangements in respect of Tri-borough Children’s Services, Tri-borough Adult Social Care and Tri-borough Public Health Services.

According to a paper on the Westminster website, the decision was urgent “because the tri-borough agreements require a year’s notice to terminate the shared arrangements; and ideally any new arrangements need to be in place before the next financial year beginning April 2018; and as soon as possible so that staff can be clearer about their future options”.

Westminster and Kensington & Chelsea said they were determined to continue to work together. They also maintained that the Tri-borough project, which was established in June 2011, had improved services and realised £43m in savings.

The two authorities stressed that Tri-borough’s legal agreements “set out that with any termination of the arrangements all parties are obliged to minimise disruption to delivery of services and to staff during the period of notice”. They called for a joint project team with Hammersmith & Fulham to oversee the transition.

The Leader of Westminster City Council, Cllr Nickie Aiken, said: “We would not have chosen to end the Tri-borough arrangements which we believe have been a great success. When it was established in 2011 it was quite rightly lauded as an innovation in local authority service delivery.

“However, both the Leader of Kensington and Chelsea and I feel we are unable to continue with tri-borough when we have a partner that we do not believe is committed to it as we are and appears to be making their own plans to leave, without any formal discussions. We can’t have that uncertainty for staff and these vital services which is why, with much regret, we have taken the very reluctant decision to terminate the joint arrangements for children’s services, adult social care and public health.”

Cllr Aiken added: “We are confident that the future remains stable and positive for the continued sharing of services between Westminster and Kensington and Chelsea and our door remains firmly open should Hammersmith & Fulham wish to come and discuss a review of the current arrangements and find alternative ways of working together.”

Hammersmith & Fulham’s Cowan said: “We’ve had concerns for some time about the value of the ‘Tri-borough’, its lack of transparency and its built-in conflicts of interest.

“In our last two budgets, Hammersmith & Fulham Council found £31m in savings but the ‘Tri-borough’ contributed no more than £200,000 of that, less than 1%.”

Cllr Cowan claimed that problems with Tri-borough contracts had cost Hammersmith & Fulham more than £5m, including a contract for special needs transport that he argued had put its disabled children at risk.

He added that “senior Tri-borough officers have had to balance Hammersmith & Fulham’s determination to keep Charing Cross Hospital open with Westminster and Kensington & Chelsea’s support for closing it.”