Alastair Campbell, who was head of communications at No 10 under Tony Blair, believes Mr Johnson’s appointment will “not end well”.
“I can think of no other senior position being filled by someone so unqualified to do the job, especially at this crucial time for the country,” Mr Cambpell said in a column in the Daily Mirror.
“I have seen close up how hard it is to do the job of Prime Minister. I have seen Boris Johnson close up.
“I see no way this ends well. I hope I am wrong. But this is one of the darkest days of my, and more importantly Britain’s, life.”