Jeremy Corbyn is already facing a backlash within Labour ranks after appointing one of his most hardline left-wing allies as shadow chancellor.Serial rebel has been handed the crucial task of overseeing the party's economic policy in a team that Mr Corbyn's aides described as "refreshing" and "dynamic".
Former leadership rival Andy Burnham has accepted a job as shadow home secretary, although a host of significant players have refused to take on frontbench roles. There has also been criticism that no women are in the most senior positions.
The scale of the problems Mr Corbyn faces were underlined as Hilary Benn, who is staying on a shadow foreign secretary, declined to offer full endorsement for Mr McDonnell's appointment.
Asked if he was 100% behind the move, Mr Benn told BBC Radio 4's Today programme: "This is the choice that Jeremy has made. I respect the choice that Jeremy has made as leader.
"We have just come off the back of two bad election defeats for the Labour Party, and our principle task is to win the people's trust when it comes to the economy.
"John's first and last task as shadow chancellor will be to win the trust and confidence of the British people in arguing for a different economic policy."