Labour leader Jeremy Corbyn is facing an open revolt after sacking Hilary Benn from the shadow cabinet.
Shadow health secretary Heidi Alexander was the first member of Corbyn's top team to quit over the dismissal.
Alexander said she was doing so with a "heavy heart" as she posted her resignation letter to Corbyn on Twitter, which stated: "As much as I respect you as a man of principle, I do not believe you have the capacity to shape the answers our country is demanding and I believe that if we are to form the next government, a change of leadership is essential."
Benn said there was "widespread" worry among Labour MPs over Corbyn's ability to win a snap election in the wake of David Cameron's resignation.
"It has now become clear that there is widespread concern among Labour MPs and in the shadow cabinet about Jeremy Corbyn's leadership of our party," he said in a statement.
"In particular, there is no confidence in our ability to win the next election, which may come much sooner than expected, if Jeremy continues as leader.
"At this critical time for our country, following the result of the EU referendum, we need strong and effective leadership of the Labour Party that is capable of winning public support so that we can stand up for the people of Britain.
A spokesman for Corbyn insisted he has no intention of resigning.
"Jeremy Corbyn is the democratically elected leader of the Labour Party and will remain so," the spokesman said.
Corbyn has pulled out of a speech at the Glastonbury Festival on Sunday, which would have seen him make his first appearance at the event. He was due to discuss the Brexit vote with revellers.