US tax authorities continue to fight legendary Irish horseracing figure JP McManus over a multimillion dollar backgammon wager.

In court papers filed this week, US federal attorneys claim that the argument made by McManus’ attorneys that their client isn’t subject to US withholding tax on gambling winnings “bends the law beyond breaking point.”

In 2012, McManus squared off against private equity billionaire Alec Gores in a three-day “serious” backgammon match in California. When the dust cleared, McManus had won $17.4m off the hedge fund boss. However, Gores held on to $5.2m of this bounty based on his belief that the proceeds were subject to US federal income tax.

Last September, McManus filed a claim in US federal court arguing that a 1997 tax treaty between the US and his native Ireland exempted his winnings from the clutches of the US Internal Revenue Service. McManus’ attorneys claimed that, for tax purposes, their client was an Irish resident in 2012 and thus only responsible for paying a flat tax of €200k.