The government’s infrastructure adviser has announced he is quitting his role, describing Brexit as a “populist and nationalist spasm”.
Lord Adonis said Prime Minister Theresa May was “pursuing a course fraught with danger” over the UK’s EU departure.
The ex-Labour minister is 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.”
The source added: “He’s now walked through the door before he was pushed.”
Lord Adonis, who was transport secretary under Gordon Brown between 2009 and 2010, has chaired the National Infrastructure Commission since 2015.
The commission produces a report in every Parliament advising the government on spending in areas like transport connections and energy.
Lord Adonis sparked anger earlier this year when he compared Brexit to the appeasement of the Nazis in the 1930s, and has repeatedly called for last year’s referendum vote to be reversed.