Ospreys will continue playing in the URC and Europe next season, but their future beyond the end of the 2026-27 campaign looks even more in doubt.
There has been ongoing uncertainty around what will happen with the £5m development St Helen’s ground in Swansea, that was set to be the Ospreys’ new home from the start of next season.
Swansea Council, which is funding the project, said its legal action would be aimed at safeguarding the future of regional rugby in the city.
In its pre-action legal letter, Swansea Council said the decision to cut the number of regions was “a restriction of competition and has not been carried out in a fair, transparent or non-discriminatory way.”
It also said that the WRU’s ownership of Cardiff created a “clear conflict of interest.”
The council has asked for a response to its pre-action letter by 13 February.
“Coming out of the meeting we were clear on what we were told, we felt the public had a right to understand what the conclusion of that meeting meant,” said Rob Stewart, leader of Swansea council told BBC Wales.
“We’ve respected parts of the meeting, but there are some clear themes around the fact the decisions seems to have been made to end regional rugby here in Swansea.
“We felt the public of Swansea deserved to know what the proposals were.
“There was not an agreement to all parts of the minutes between the Ospreys and the WRU.
“So we felt it better to put out a statement that was not challengeable in terms of legal aspects, rather focus on the main themes and conclusions, which are not in doubt.
“We’re continuing to support the Ospreys, to continue seeing them play regional rugby here in Swansea, but that does mean having a legal challenge towards the WRU and Y11.”
When asked whether a legal challenge was the best use of public money, Stewart said: “For our region here, there’s in excess of £12m of direct impact to the economy, 20,000 people play rugby in Swansea every week.”
“All of that is at risk so it is in the public interest, and as a public body we have to stand up for what’s right.
“We will continue to fight on behalf of the people of Swansea.”
