The former head of GCHQ has urged foreign secretary Philip Hammond to publish secret guidelines governing how ministers share intelligence that helps the US kill militants with drone strikes.

In a letter seen by the Financial Times, Sir David Omand, the former director of the government monitoring agency, has called on Mr Hammond to release the guidelines, the existence of which has never been formally acknowledged.

Sir David says that publishing such information would help allay concerns among British citizens that the government is breaking the law by assisting US lethal drone strikes outside traditional battlefields. The letter says: “We believe there is an increasing public interest in the disclosure of the guidance and restrictions concerning the passing or use of UK intelligence.” It has also been signed by Tom Watson, the Labour MP, David Davis, the Conservative MP, Michael Clarke, the head of the Royal United Services Institute, and Baroness Stern, the prison reformer who is interested in the subject of drones.