Newport Gwent Dragons say Taulupe Faletau's decision to join Bath at the end of the season is "the worst possible" outcome to his transfer saga.
Faletau, 25, has rejected an offer of a national dual contract (NDC) with the Dragons and the Welsh Rugby Union.
Dragons chief executive Stuart Davies said he is "extremely disappointed" by the decision and questioned the WRU's handling of the negotiating process.
The WRU says it liaised with Dragons "on all developments" during talks.
But Davies said: "I want to fully review how the negotiation was handled. I need to understand it for my own peace of mind.
"And in a wider context I'm tired of Toby and the Dragons being a test case for these policies, procedures and documents."
Bath head coach Mike Ford confirmed that Faletau had signed a three-year deal on Wednesday.
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Loss of a 'significant' transfer fee
Faletau had been set to join Bath in October but a move was blocked by Wales head coach Warren Gatland.
The proposed transfer would have involved a fee because the number eight was under contract, but he will now join the English Premiership side for free when his deal ends at the end of the season.
"It's the worst possible scenario for the Dragons as an outcome," said Davies.
"The value of the transfer was significant. Some of the figures I've seen bandied around didn't do justice to what it meant to us in terms of a figure.
"And obviously the NDC not going through now is the knockout follow-up."
Davies added that the outcome vindicates their decision to attempt to ensure a transfer fee for a key member of the Wales squad.
When asked if they would attempt to scupper the move to Bath by selling the player to another club, Davies said: "The process surrounding that remains the same.
"It would see us having to go to the Union and gain Warren Gatland's permission. So if Warren's position on that hasn't changed then we won't be pursuing it."
Faletau in demand
Faletau has been with the Dragons since 2009 and has won two Six Nations titles with Wales, including one Grand Slam in 2012.
The Tonga-born back-rower has been capped 52 times by Wales, and represented the British and Irish Lions on the 2013 tour of Australia.
Following the collapse of his proposed transfer to Bath in October, Faletau indicated he was keen to stay with the Dragons on a dual contract.
Negotiations with the WRU soon began, although Bath remained determined to tempt him over the border.
Davies said he was under the impression Faletau was still considering the offer of a dual contract and so was "extremely surprised" to learn of his decision to join Bath on Wednesday.
"Naturally it's disappointing, it does raise questions about the NDC process," he said.
"The inability to reach a satisfactory conclusion on this focuses attention on the NDC process.
"There are players signed up to an NDC that don't have Toby's standing in the game and naturally we're very disappointed that we haven't been able to secure his services."
The one that got away
To date, the WRU have signed up 18 players on dual contracts, which are funded 60% by the WRU and 40% by the region.
The initiative has helped secure some of Wales' leading players in Wales, while also bringing others home from French clubs.
On Tuesday, the WRU revealed an offer to Scarlets centre Scott Williams has been withdrawn after regions had "upped the ante" over the Wales international's contract.
In response to Davies' criticism of the process with Faletau, the WRU said it is "disappointed but accepts the player's decision in which he identified a desire to pursue a new rugby challenge at this stage of his career".
The statement added: "It remains the policy of the WRU to keep as many Welsh qualified players as possible in Wales with one of the four regions."