Brussels - Germany's fractious interior minister, Horst Seehofer, provoked irritated reactions in Brussels on Friday with a letter in which he warns that the European Union's tough position on Brexit could endanger the safety of citizens.

The letter is a sign of possible tensions under the surface, as the 27 EU member states remaining after Britain's departure in March have sought to present a united front in dealing with London.

"Security of citizens is highest priority"

"There is only one Brexit negotiation and one Brexit negotiator; this is Michel Barnier," European Commission spokeswoman Natasha Bertaud stressed on Friday.

Seehofer's argument is "not the position of the European Council, including Germany," a spokesman for Barnier told dpa, referring to the official formation of EU leaders.

"We have a mandate from the European Council which we plan to respect," European Commission President Jean-Claude Juncker added.

Bertaud confirmed receipt of the letter, first reported on by the Financial Times and seen also by dpa, in which Seehofer argues that the security of citizens should "not be up for discussion."

"It is not my intention to comment on the commission's negotiations with the United Kingdom and their strategic direction," Seehofer wrote.

"I allow myself however, as the interior minister of a European member state, to draw attention to the fact that the security of citizens must also be of the highest priority in the EU," he added.

His argument is that being used by Britain, while the EU insists that the current structures for security cooperation rely on being part of the EU's legal and political framework.

At stake are issues such as extradition agreements or shared security databases to track down criminal suspects.