The European Commission has said the UK crashing out of the EU without a deal is now a "distinct possibility".
"Our working assumption is that there will be Brexit on October 31," spokeswoman for the Commission Mina Andreeva told reporters.
"Whether it is the most likely scenario... I would say it is a very distinct possibility, which is precisely why we launch this final call to be prepared in case a no-deal Brexit occurs."
Ms Andreeva said that EU leaders still need concrete proposals to replace the Irish backstop.
The German Chancellor last month said if the UK could find a workable alternative to the backstop she would listen, while Mr Johnson welcomed the "blistering timetable of 30 days" and agreed the "onus is on us to produce those solutions".
Ms Andreeva also said officials still believe the UK leaving the EU on the basis of the deal agreed by Theresa May - a deal rejected by Parliament three times - would be the best solution.
She added: "The best outcome would be a Brexit on the basis of the negotiated withdrawal agreement."
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