The UK must submit a written proposal for a Brexit deal by September 30 or "it's over", Finland's Prime Minister Antti Rinne has said.

Johnson met the EU's Jean-Claude Juncker on Monday (Photo: AP)

Rinne, whose country currently holds the EU's rotating presidency, spoke after a meeting with French President Emmanuel Macron.

According to Helsingin Sanomat, he told Finnish journalists that if the UK wanted to discuss alternatives to the previous Brexit agreement "these must be presented before the end of the month."

"If not then, it's over", he said.

UK Prime Minister Boris Johnson has said that the UK is going to leave the EU on October 31, with or without a deal.

He has said that he would rather "die in a ditch" than agree to a fresh delay to Brexit.

Johnson says he is still hopeful of striking a Brexit deal with the EU which can get the support of MPs - his predecessor Theresa May's agreement with the EU failed to win a majority in the House of Commons.

But Mr Rinne's comments have raised the stakes although the deadline is not yet an official one from the EU side. Rinne is set to discuss it with European Council President Donald Tusk and Johnson in the days ahead.

September 30 comes in the middle of the annual gathering of Johnson's Conservative Party. It has been widely suggested that Johnson planned to wait until after the conference before unveiling any modified Brexit deal proposals, with the mid-October EU summit seen as the crunch time for any outline of a deal to be agreed.

Downing Street says it is continuing to negotiate and will put forward its proposals "at an appropriate time."

The UK is leaving the European Union after a referendum in 2016 saw people vote by 51.9 percent to 48.1% to leave.

However the terms of the UK's departure and its future relations with the EU has yet to be agreed, with what happens on the Northern Ireland/Republic of Ireland border the main sticking point.