BRUSSELS—Britain will have less than 18 months to negotiate its exit from the European Union once talks begin and won’t be allowed to pick and choose what it likes, the EU’s chief Brexit negotiator said Tuesday as he outlined his stance for the first time.

Michel Barnier said formal procedures like parliamentary approvals across Europe will cut into the two-year period that Britain was expecting to have to negotiate the terms of its exit.

“Time will be short,” Barnier said in Brussels. “We are ready; keep calm and negotiate.”

British Prime Minister Theresa May has voiced her intention to invoke by the end of March Article 50 of the EU Treaty, the step that will officially kick off two years of exit talks.

But Barnier warned that that the effective negotiating time will be less. An agreement, he says, may have to be secured by October 2018 to get an agreement in place by March 2019 — two years on from the triggering of Article 50.

May could even struggle to stick to her timetable of invoking Article 50 after a court last month decided that Parliament had to give its backing to such a move. The government is contesting that ruling in the Supreme Court. Should it confirm that Parliament has to be involved, it could delay May’s plans to begin the exit discussions.

Barnier, who spoke in English and French, said that Britain can’t “cherry-pick” what it likes about the EU, noting says that the single market and its four freedoms, such as the freedom of movement, are “indivisible.”

The Frenchman did not want to be drawn on the possibility of a transitional deal, in which Britain retains partial access for a period of time to ease its exit.

“It is for the British to say what kind of relationship they want and for the 27 EU states to define the future they want to build with them,” he said. “You can’t do everything in 15 to 18 months of negotiations”

The British government has been reluctant to reveal much about what it wants from the discussions and what sort of post-Brexit relationship it is looking for out of fear that it would weaken its hands in negotiations.

Earlier, Britain’s finance chief said all options remained on the table, including the possibility that the country may continue paying into EU coffers for access to the single market after leaving the bloc.

In a rare appearance for the regular meeting of the EU’s 28 finance ministers, Philip Hammond stressed that he wants to minimize the economic disruption from Britain’s exit. Leaving the EU single market would be a blow to many British businesses that would face new tariffs on trade.

“I think that it’s in everybody’s interest on both sides of the English Channel to have a smooth a process as possible that minimizes the threat to European financial stability and minimizes the disruption to the very many complex relationships that exist between European manufacturing businesses and their financing banks and so on in London,” he said.

That, he conceded, may include ongoing contributions to the EU budget after so-called Brexit.

In terms of any future contributions to particular organizations, Hammond said “we would look at what was on offer, we would look at the cost, and as you do with anything we would decide if that was a good deal or not.”

Though there’s been a degree of understanding across European capitals about the constitutional arrangements that have to play out in Britain, there’s also been discontent over the approach the British government has taken so far. One idea that’s vexed politicians in Europe is that Britain can “have its cake and eat it,” — that it can still have access to the single market while applying stricter immigration controls. The EU stresses that the freedoms of trade, money and people are inseparable and a country cannot pick and choose.

Jeroen Dijsselbloem, the Dutch finance minister who also chairs meetings of the 19 EU countries that use the euro, said that the proposals he’s seen so far “are incompatible with smooth and incompatible with orderly.”

“It can be smooth and it can be orderly but it requires a different attitude on the part of the British government,” he said. “If the U.K. wants to have full access to the internal market they will have to accept the rules and regulations that go with that internal market.”