Darling, who along with Jack Straw and Gordon Brown was one of just three surviving cabinet ministers from Tony Blair’s original line-up in 1997, said it had been an “enormous privilege” to serve in government.
He added: “But I believe it is time for me to return to the back benches, from where I shall look after, with great pride, the constituents of Edinburgh South West.”
The former chancellor also refused to back any of his colleagues in the race to become the next Labour leader.
“I shall be on the back benches by the time the new leader takes up office, so I have plenty of time to reflect on these things,” he said.
When in opposition, the shadow cabinet is voted in by party members, but a spokesperson for Darling said he would not take part in the elections.
It would have been extremely difficult for Darling to stay on as shadow chancellor. With the Tories growing ever-angrier about their economic inheritance, he would have become a lighting rod for their attacks.
Meanwhile, left-winger Jon Cruddas ruled himself out of the running for the Labour leadership, saying he had no designs on the top job.