The intervention of the attorney general in the botched Renewable Heat Incentive (RHI) scheme has taken the scandal to a "new level of dysfunctionality", the Ulster Unionist leader has said.
John Larkin, the Northern Ireland Executive's top legal adviser, said he is considering challenging the legality of the RHI scheme that could cost taxpayers £490m.
Mr Larkin revealed he may bring a case against the Department for the Economy for introducing the scheme before it went to the full executive.
The attorney general made the comments during a separate case at Belfast's High Court.
The development came as more than 500 members of the Renewable Heat Association NI Ltd were granted leave to seek a judicial review of Economy Minister Simon Hamilton's plan to cut tariff payments.
However, Ulster Unionist leader Mike Nesbitt criticised Mr Larkin's intervention in the scandal, saying it was "bizarre" that the Executive, "through its top legal adviser, was suing itself".
"This is dysfunctionality on an unprecedented scale," said Mr Nesbitt.
He said the intervention adds "yet another layer to this fiasco".
"What are we paying all these departmental solicitors for if it gets to this point? They are meant to ensure that our devolved government makes regulations which are lawful and not wide open to challenge.
"The intervention of the attorney general in RHI takes this to a whole new level of dysfunctionality.
"It is already clear that any suggestion that the minister for the economy has produced sound solution to the massive overspend on RHI, even in the short term, looks increasingly fanciful.
"All the signs are that the whole Renewable Heat Incentive is being tied up in legal knots."
The RHI scheme was an attempt by the Northern Ireland Executive to help to increase creation of heat from renewable sources.
But flaws in setting the scheme's subsidy rate left it open to abuse as claimants could earn more cash the more fuel they burned, with the overspend estimated to be about £490m.