Finnish Prime Minister Antti Rinne says 'If not by then, then it’s over.' | ANTTI AIMO-KOIVISTO/AFP/Getty Images Finnish PM: UK must make Brexit proposal by September 30 EU countries are keen to avoid a last-minute negotiation at the European Council summit in mid-October.

PARIS — Finland wants the U.K. to submit written proposals for a Brexit deal by September 30 or it will be "over," according to Finnish press reports.

“If not by then, then it’s over,” Finnish Prime Minister Antti Rinne told Finnish reporters in Paris on Wednesday, after meeting with French President Emmanuel Macron at the Elysée. Finland currently holds the rotating presidency of the Council of the EU.

Rinne's comments look designed to increase pressure on U.K. Prime Minister Boris Johnson, who European officials say, still has not proposed any concrete alternatives to the Irish backstop that has been a main sticking point in the Brexit negotiations.

The September 30 date echoes a timeline set out by German Chancellor Angela Merkel and Macron at the end of August when Johnson visited both Berlin and Paris. Macron spoke at the time of the need for a "useful month."

The warning is meant to avoid a last-minute negotiation at the European Council meeting set to take place on October 17 and 18, two weeks before the official deadline for the U.K. to leave the EU.

“We have to have proposals quickly and have the time to run expertise. We won’t negotiate in real time at the European Council," said a French diplomatic official.

A September 30 deadline will be difficult for Johnson. The Conservative Party conference is set to kick off on September 29 and putting forward a Brexit offer ahead of that risks precipitating a split at the party gathering.