Theresa May's Government is putting a successful Brexit at risk by failing to show "active and energetic" leadership, the head of the National Audit Office has warned.

In an unprecedented intervention Sir Amyas Morse said Whitehall departments are being left to "struggle on their own" with the challenges of Brexit because of a failure of leadership and direction.

He said the Government could "come apart like a chocolate orange" unless departments are given more support, as he warned Mrs May that Brexit poses the "biggest challenge" since the Second World War.

Departments should not be left to decide which of the many projects they are in charge of to prioritise by themselves, the head of the financial watchdog added.

Sir Amyas also revealed that David Davis's Brexit department failed to show him a plan for how leaving the EU will work, despite his requests, and could only offer a "vague" explanation as to why it was unable to.