Christchurch Borough Council has voted to join a committee set up to discuss a possible merger of Dorset's councils, despite opposing the plans.
In February, councillors voted against forming a unitary authority with Bournemouth and Poole in a proposed reorganisation of local government.
But now councillors have voted unanimously to join the new committee.
Councillor Claire Bath said not engaging in discussions would "disadvantage staff and residents".
At the meeting, councillors were told the cost of joining the committee would be about £80,000.
The proposal involves merging Dorset's nine councils into two unitary authorities, the first made up of Bournemouth, Christchurch and Poole, and the other made up of East Dorset, North Dorset, Purbeck, Weymouth & Portland and West Dorset.
The plan aims to save about £108m over six years.
Six councils have backed the plans and submitted their proposal to Communities and Local Government Secretary Sajid Javid.
But Christchurch, along with Purbeck and East Dorset, voted against the changes earlier this year.
Purbeck and East Dorset have already announced they will take their seats on their area's joint committee.
Christchurch said it would hold a referendum for residents.
If the merger is approved it would come into effect from April 2019.