While Johnson and his ministers have in recent days talked up progress in negotiations with Brussels, the EU side has consistently sounded less optimistic, emphasising that Britain must come up with new ideas.
In an interview with Deutschlandfunk radio on Sunday, Juncker said there was no possibility of reopening the Withdrawal Agreement negotiated by May.
"I believe we need to maintain a decent relationship with Britain, and I am not optimistic when it comes to finding alternative arrangements that will allow us to limit the Irish backstop," Juncker said.
"We do not know what the British want in detail, precisely and exactly, and we are still waiting for alternative proposals. I hope we can get it, but time is running out."
Johnson's government has yet to publicly announce new proposals on how to solve the Northern Irish border issue. It insists that the backstop, an insurance policy to prevent a return to a hard border, is unacceptable.
Johnson has pledged to leave the EU with or without a deal on Oct. 31, even though British lawmakers have passed a law which would force him to request a delay beyond that date if he is unable to reach a deal with the EU. His government has stepped up preparations to mitigate possible food, fuel and medicine shortages in case of a no-deal exit.
"We want a deal. This cannot include the backstop," the British government source said, adding that Johnson will make clear to Juncker that even if the EU offers him an extension of the Brexit deadline beyond Oct. 31 he will reject it.
