German Foreign Minister Heiko Maas Maas | Ludovic Marin/AFP via Getty Images Heiko Maas: UK must pledge ‘zero unfair competition’ to get ‘zero tariffs’ Brexit trade deal German Foreign minister says consumers and the environment must not be neglected by a post-Brexit trade agreement.

Punishing the U.K. for Brexit would be a "grave mistake," but the EU will need to be "politically wise and fair" in the next stage of negotiations "in a way that wouldn’t harm the European Union," German Foreign Minister Heiko Maas has said.

In an open letter to the British people, published today, Maas writes: "Yes, we all want zero tariffs and zero trade barriers, but that also means zero dumping and zero unfair competition. Without similar standards to protect our workers, our consumers and the environment, there can be no full access to the largest single market in the world."

He also warns that time is short to agree on a deal, calling it a "Herculean task," while the list of things to negotiate is long, including "free trade, fair competition, visa-free travel, work and travel opportunities, university exchanges, protections against terrorism and organised crime, management of international crises."

Acknowledging that "things haven't always been easy, especially since the Brexit referendum," Maas expresses his wish for a new partnership "as close as possible" between the U.K. and the EU. "Ultimately, we share this continent and also the same European values."

However, he warns that things cannot stay the same as when the U.K. was a member of the bloc. "It [the relationship] will have to be looser than it was within the EU, but how loose it becomes is up to us."

Hinting at a possible future reentry of the U.K. into the EU, Maas writes that "rest assured that we will always have a place for you at our table in Brussels and in our hearts."

Ending with a reference to the Beatles' song "Hello, Goodbye," he says: "We have sorted out the goodbye ... So let’s sort out the hello, our future. Separate, but together."