Anti-gay activist Brian Brown said the rights commission was an "extraordinary opening."

The National Organisation for Marriage has been denied a reimbursement of $690,000 in legal fees, after a failed lawsuit against the IRS.

The anti-gay group sued the Internal Revenue Service last year, alleging that in 2012 it had disclosed details of donations from a political action group linked to then-Republican presidential nominee Mitt Romney.

NOM had claimed that the leak was politically motivated and deliberate, but US District Court Judge James Cacheris ruled earlier this year that NOM had “failed to produce a shred of proof” for the claims, dismissing most of the allegations.

The IRS eventually agreed to settle the one remaining count out-of-court for $50,000 (£31,000) in damages – but court filings this week reveal that the group sunk nearly fourteen times that amount into fighting the failed legal battle.

According to documents released by Good As You, Judge Cacheris this week refused to reimburse the group $691,025.05 (£430,000) in legal fees.

He wrote: “The end result of this litigation is exactly what NOM already knew as early as December of 2013, if not prior: the IRS inadvertently disclosed a single copy of an unredacted form.

“Subsequently, with all other claims dismissed, the parties settled for $50,000, inclusive of actual damages and costs related to that single disclosure.

“NOM’s attempt to conflate this outcome as the primary or most significant issue does not change the fact that, at heart, this case presented a series of claims against the IRS, the vast majority of which were determined in the IRS’s favour on summary judgement.

“Accordingly, the court need not address the reasonableness of NOM’s $691,025.05 fee request.”

As the anti-gay group does not disclose its list of donors, it is unknown who funded the legal battle.