LONDON, United Kingdom (AFP) — The EU’s chief Brexit negotiator will hold talks with British Prime Minister Theresa May in London on Monday as the two sides resume negotiations on post-Brexit ties in a climate of deep uncertainty.

“The prime minister will welcome Michel Barnier to Downing Street on Monday,” read a tweet on the official Downing Street account on Friday.

Barnier and the UK’s Brexit Minister David Davis “will discuss the upcoming negotiations on the implementation period and the next steps in building a new partnership between the EU and the UK,” it said.

The European Commission said there would also be negotiations between British and EU officials in Brussels from Tuesday to Friday and these would include a “UK update on the future relationship”. Britain has said it wants to leave the European single market and customs union and retain strong economic ties with the EU but has given little detail.

Downing Street on Friday rejected a Financial Times report that ministers are privately considering a customs union deal for post-Brexit trade.

In an interview in Shanghai with Bloomberg TV, International Trade Secretary Liam Fox also ruled this out saying: “We have to be outside of that to take advantage of those growing markets.” “One of the reasons we are leaving the European Union is to take control and that’s not possible with a common external tariff.”

Britain voted to leave the European Union in a momentous referendum in June 2016 that bitterly divided the nation, sparked tumultuous period in British politics and caused global shockwaves. London and Brussels struck a preliminary deal on key Brexit issues in December but are yet to discuss the conditions for a post-Brexit transition period and future trade relations between Britain and the EU.

May has faced growing public calls from across her Conservative party this week to set out her vision for Brexit more clearly, amid media reports of increasing discontent with her leadership. Tensions were fuelled by a leaked internal government assessment earlier this week suggesting that the options on the table so far for a trade deal with the EU would all leave Britain worse off.

The prime minister also sparked concern in Brussels by questioning the rights of EU citizens who arrive in Britain during the post-Brexit transition period.

In a BBC interview at the end of a three-day trip to China, May declined to detail exactly what she wants from negotiations on future ties.

Asked whether she would favour a deal that limited disruption to trade, or one that emphasised Britain’s power to forge its own path, she said: “These are not the options we have before us.

“The option that we have before us is to go in there with the EU and negotiate a good trade deal. “What the British people voted for is for us to take back control of our money, our borders and our laws and that’s exactly what we are going to do.”

Sensing a lack of coordination in government on Brexit, pro-EU politicians have become increasingly vociferous in Britain in recent days.

Labour MP Chuka Umunna, head of a new coalition of pro-EU groups formed this week, told AFP on Friday that the government was “trying to deliver something that isn’t deliverable.”

“If the people started this in 2016, they should be the ones who decide how we end it, and whether or not we do actually leave,” Umunna told AFP while out campaigning in Brixton in south London