London, Dec 30 (IANS): One of British Prime Minister Theresa May's key advisers, Lord Andrew Adonis, has announced he is quitting his role, describing Brexit as a "populist and nationalist spasm".

Adonis, who was appointed as her infrastructure adviser in 2015, said May was "pursuing a course fraught with danger" over the UK's EU departure, the BBC reported.

The one-time Secretary of State in Tony Blair's Labour government resigned as chair of the National Infrastructure Commission. He was already a high-profile campaigner against Brexit.

A government source said: "He's been moving closer towards the exit door with each new onslaught he makes against Brexit. He's now walked through the door before he was pushed."

Adonis later insisted it had been his decision to leave, as his "differences with the government had become too great".

In his letter to 10 Downing Street, Adonis, who would now sit in the unelected House of Lords, told May: "The European Union Withdrawal Bill is the worst legislation of my lifetime.

"It arrives soon in the House of Lords and I feel duty-bound to oppose it relentlessly from the Labour benches."

He told May her government "is hurtling towards the EU's emergency exit with no credible plan for the future of British trade and European cooperation".

He also claimed that Brexit was causing a nervous breakdown across Whitehall, Xinhua news agency reported.

He said: "If Brexit happens, taking us back into Europe will become the mission of our children's generation, who will marvel at your acts of destruction."

There was no immediate official response from 10, Downing Street.

Leading Brexiteer, Conservative MP Iain Duncan Smith said the departure of Adonis was "long overdue".