In a leaked resignation letter Britain’s outgoing former civil service chief in Brussels, Sir Ivan Rogers, gives a withering assessment of the government’s approach to free trade, which has been widely seen as a direct attack on International Trade Secretary Liam Fox.

In a lengthy resignation announcement, emailed to Foreign Office staff in Brussels, Sir Ivan said: "Contrary to the beliefs of some, free trade does not just happen when it is not thwarted by authorities. Increasing market access to other markets and consumer choice in our own depends on the deals, multilateral, plurilateral and bilateral, that we strike and the terms that we agree."

Dr Liam Fox has been criticised for his pronouncements on free trade, particularly in his party conference speech on the ‘glorious joy of free trade,’ which argued government’s only role is in removing barriers to trade.

Farage calls for more resignations after Sir Ivan Rogers quits

It has been reported that Sir Ivan threatened to resign several times during David Cameron’s re-negotiation with the EU a year ago, when he said his suggestions were not being listened to.

In his letter, Sir Ivan added: "I hope that you will support each other in those difficult moments where you have to deliver messages that are disagreeable to those who need to hear them. I hope that you will continue to be interested in the views of others, even where you disagree with them, and in understanding why others act and think in the way that they do.

"I hope that you will always provide the best advice and counsel you can to the politicians that our people have elected, and be proud of the essential role we play in the service of a great democracy."

The letter also warned: "Serious multilateral negotiating experience is in short supply in Whitehall, and that is not the case in the [EU] Commission or in the [EU] Council.”