Sir Tim Barrow, Britain’s permanent representative to the European Union, handing the Article 50 letter to Donald Tusk, president of the European Council, yesterday Yves Herman/EPA

Theresa May was accused last night of blackmail after she warned that the fight against crime and terrorism would be undermined if the EU refused to strike a Brexit deal with Britain.

The prime minister adopted a conciliatory tone as she triggered Article 50 yesterday. However, she said that the consequences of the talks ending in failure would cause more than economic damage to both sides.

“In security terms a failure to reach agreement would mean our co-operation in the fight against crime and terrorism would be weakened,” she wrote in her letter to Donald Tusk, president of the European Council.

It drew an angry response from MPs, EU negotiators and diplomats. “Our security is far too important to start bargaining it against an economic…