Theresa May has delivered a “blatant threat” to the EU 27 by threatening to withdraw security co-operation if we do not receive a favourable trade deal, Liberal Democrat leader Tim Farron has said.

The letter comes as a European Parliament text outlining the EU’s position has warned the UK not to seek a trade-off between security and trade in the Brexit negotiations.

Tim Farron said: “It is shameful that Theresa May has threatened to withdraw security co-operation from our closest neighbours and allies. With growing terrorist threats from around the world, it is imperative that we work together with European allies for our mutual security. She is prepared to put the safety of British and European citizens on the line just so she can deliver her hard Brexit.

“Security is too important to be used as a bargaining chip and this will backfire in any negotiations, which rather than building up alliances will leave Britain even more isolated.”

Tim Farron was also deeply concerned by what May’s letter had to say about trade.

He said: “Theresa May admits that we will lose influence over the very rules by which British firms exporting to our largest market will have to abide by. So much for taking back control. The Conservative Party used to be the party of business, but by going for this reckless hard Brexit she has thrown British business into a state of uncertainty and powerlessness.

“This is Theresa May’s choice to go for a hard Brexit. It wasn’t on the ballot paper, and the Conservatives in their election manifesto said we should stay in the Single Market. That is why we will continue to fight to give the people a say on the final deal.”

Theresa May’s Article 50 letter contains a clear threat that future security cooperation between the UK and EU will be contingent on getting a good trade deal, wanting to take account of “both economic and security cooperation”, and warning that failure to reach a deal would mean “cooperation in the fight against crime and terrorism would be weakened.”

The word security is mentioned eleven times – and trade just six times – in Theresa May’s Article 50 letter.