Ireland's Conor McGregor has told Dustin Poirier their third fight scheduled for July 10 is off in a Twitter spat over a $500,000 donation to the American's charity.

Former two-weight UFC champion McGregor said earlier this month that a third fight was booked with Poirier, who became the first man to knock him out in January. McGregor knocked Poirier out in a featherweight contest in 2014.

But McGregor was not happy when Poirier said the Irishman had not made good on a promise to donate to his charity.

"A donation, not a debt. We've been awaiting the plans for the money that never came," McGregor said in an expletive-laden reply.

"500k with no plan in place ... You must be new to money.

"The fight is off ... I'm going to fight someone else on the 10th."

Poirier replied by saying McGregor's team "never responded" to emails explaining where the funds would go.

McGregor's last UFC victory was in January 2020 and he came out of retirement to face Poirier again after winning their first contest in 2014.