European Council President Donald Tusk met the British prime minister during the UN General Assembly | Sergei Supinsky/AFP via Getty Images Tusk and Johnson will meet Sunday for Brexit pow-wow Meeting on the sidelines of the G7 will be their first since British PM demanded the EU renegotiate Brexit deal.

European Council President Donald Tusk and U.K. Prime Minister Boris Johnson will meet Sunday in Biarritz, France to discuss Brexit on the sidelines of the G7 summit, a senior EU official said.

It will be their first face-to-face encounter since Johnson demanded that the EU renegotiate the Withdrawal Agreement and discard the backstop provision regarding the Irish border.

Tusk swiftly rejected Johnson's demand in a statement that implicitly accused the prime minister of not admitting the true ramifications of eliminating the backstop, which in the EU's view would effectively necessitate re-establishment of a border between Ireland and Northern Ireland, and thereby undermine the Good Friday Agreement.

But as the Brexit debate becomes as much about political stagecraft in the weeks leading up to the October 31 deadline for the U.K.'s departure, the real question on Sunday may be: How much oomph can Tusk take?

Johnson on Tuesday said he would approach his talks with the EU "with a lot of oomph" despite Brussels being a "bit negative."

"At the moment, it is absolutely true that our friends and partners are a bit negative," Johnson said, adding: “I saw what Donald Tusk had to say, and it wasn’t redolent of a sense of optimism.”

Johnson is due to meet German Chancellor Angela Merkel and French President Emmanuel Macron before the start of the summit.

The senior EU official said that Tusk and Johnson would discuss progress at the G7 summit but that Brexit was also certain to be raised in the conversation. Ahead of his meeting with Johnson, Tusk will meet with the EU's chief Brexit negotiator, Michel Barnier, on Thursday.

Tusk has repeatedly called Brexit a "lose-lose" proposition for the EU and the U.K. and has expressed his personal hope that Britain would reverse course, while also acknowledging that prospects were dim for such an about-face.

Tusk's term as Council president ends at the end of November, so unlike Commission President Jean-Claude Juncker, he will still be in office on November 1 — the day after Johnson has threatened to take Britain out of the EU "do or die."