President of the European Commission Jean-Claude Juncker, in Berlin | Tobias Schwarz/AFP via Getty Images Juncker grades UK’s Brexit papers an F Speaking to top EU diplomats, the Commission president insists divorce must be settled before discussion of future relationship.

European Commission President Jean-Claude Juncker sharply criticized the U.K. on Tuesday for its handling of Brexit talks, saying none of its position papers was satisfactory.

Speaking as Brexit negotiators resumed formal talks in Brussels, Juncker also rebuked London for trying to fast-forward the discussions to the future relationship rather than first settling the terms of its withdrawal from the EU.

“The U.K. government is hesitating to show all its cards,” said Juncker, speaking in French at an annual conference of the EU's top diplomats.

On Monday, the U.K.'s top Brexit official, David Davis, said the recently released British position papers on issues like citizens' rights and the border between Northern Ireland and Ireland were "the products of hard work and detailed thinking."

But Juncker, who said he had read all of them with great interest, told the ambassadors: “None of those is actually satisfactory, so there is still an enormous amount of issues which remain to be settled.”

Juncker also upbraided Prime Minister Theresa May's government for trying to prioritize discussion of the post-Brexit relationship, which the EU has insisted cannot happen until there is "sufficient progress" on divorce terms.

Juncker said Brussels would not deviate from the position agreed by the 27 remaining EU countries. “We need to be crystal clear that we'll start no negotiations on the new relationship, particularly the new economic and trade relationship, between the U.K and the EU, before all these questions are resolved," Juncker said, adding: "We cannot mix these issues up.”

May's government has tried to demonstrate that it is almost impossible to resolve the withdrawal issues, particularly matters like the border between Northern Ireland and the Republic of Ireland, without also addressing future trade and customs arrangements. Juncker conceded on Tuesday that these issues were linked to an extent, but nonetheless said the negotiations must proceed in careful sequence.

“I know there is some partial overlapping between issues but the European Council, following upon the Commission's proposal, has been crystal clear," he said. "First of all, we settle the past before we look forward to the future.”

The third formal round of negotiations kicked off in earnest Tuesday, with working groups returning to the bargaining table at the Commission headquarters in Brussels.

Juncker's criticism of the British position papers echoed those of the EU chief Brexit negotiator, Michel Barnier, who Monday said he had read them “very carefully" but there was still not sufficient clarity from the U.K. in the talks.