The European Union’s chief Brexit negotiator, Michel Barnier, accused the U.K. of not “negotiating seriously” over its withdrawal from the bloc and called on the British government to remove ambiguity about its stance. As Barnier and Brexit Secretary David Davis opened the third round of talks on the U.K.’s exit from the European Union in Brussels on Monday, Barnier expressed his mounting frustration at the slow progress of talks. The EU believes the U.K. hasn’t been transparent about what it really wants and isn’t negotiating honestly even after publishing a flurry of papers on its positions in some areas. “We must start negotiating seriously; we need U.K. papers that are clear in order to have constructive negotiations,” Barnier told reporters. “I’m concerned.” With eight weeks to go until a key EU summit at which the bloc’s leaders will be asked to judge whether the negotiations have made “sufficient progress” to allow the U.K. to open trade discussions, the talks have made little headway on Britain’s financial settlement and the Irish border. The U.K.’s bill remains the biggest stumbling block to an agreement, with British negotiators determined not to reveal where they admit the U.K. has obligations. Earlier this month Davis told the BBC that it was part of his negotiators’ “constructive ambiguity” approach to try to obtain a better deal. Financial Settlement The EU says it must agree to a methodology for calculating the financial settlement before discussions can move on. “For the U.K., the week ahead is about driving forward the technical discussions across all the issues,” Davis told reporters. “But in order to do that we’ll require flexibility and imagination from both sides.” He said the U.K.’s position papers were “the product of hard work and detailed thinking that has been going on behind the scenes, not just the last few weeks but for the last 12 months.” Neither the U.K. nor the EU is expecting much of a breakthrough during this week’s negotiations. Monday evening is devoted to the meeting between Davis and Barnier. With Davis scheduled to return to London on Monday night, technical teams will be left to seek progress in the three priority areas. Britain needs “sufficient progress” on the financial settlement, on how to keep the Irish border open after Brexit and on the status of citizens living in each other’s countries before the EU will allow the U.K. to open discussions on a future trading arrangement. Increasingly Irritated Since the last round of talks in mid-July, EU officials have become increasingly irritated by what they see as the U.K.’s determination not to engage fully with the negotiations. Despite Barnier’s repeated warning that “the clock is ticking” for the U.K. to get a deal, an EU official said last week that it was a lack of substance from the British side rather than time that was the biggest threat. For it’s part, the U.K. accuses the EU of not showing flexibility and willingness to deviate from prepared positions, including on its refusal to discuss the shape of the future relationship now. Reports on Monday that France was leading a group of EU nations who were already willing to begin trade talks as early as October if the U.K. sought a three-year transitional arrangement were dismissed by the French presidency. “France fully supports Michel Barnier’s negotiating mandate as far as the method and fundamentals are concerned,” the French presidency said via text message from a press officer.