Swansea Council has applied for a High Court injunction to halt the proposed deal between the Welsh Rugby Union and Y11 to buy Cardiff Rugby.
If the deal goes ahead it would lead to the end of the Ospreys and men’s professional regional rugby in Swansea.
The Council is seeking an urgent court hearing which could be as soon as this week.
However, if, in the meantime, the WRU provides a written undertaking that it will not complete the deal the court hearing could be delayed to a later date.
A Swansea Council spokesperson said: “The proposed deal between the WRU and Y11 to buy Cardiff Rugby would mean the end of the Ospreys as a men’s professional regional rugby team playing in the URC.
“We are seeking an injunction to prevent this deal going ahead so that further discussions can take place while the current four‑team regional structure remains in place.
“The injunction would give all parties the opportunity to consider other options for the future of regional rugby in Wales.
“We believe this is a reasonable and proportionate action given the risk to the future of the Ospreys and the impact that would have on Swansea and the wider region.
“Players, supporters, residents, community clubs and local businesses stand together in support of the Ospreys remaining as one of Wales’s four professional regional rugby sides.”
The decision to apply for an injunction was taken by the Council’s Cabinet on Monday and had all-party support across the Council.
In a separate action earlier this week, the Council formally asked the Competition and Markets Authority to investigate the proposed deal between the WRU and Y11.
The Council’s concerns focus on the proposed reduction of professional teams from four to three, the process for allocating licences, and the potential for one ownership group to control two Welsh clubs.
