The former chancellor – MP for Edinburgh South West – hailed Mr Miliband's "Labour values", capacity to inspire and ability to unite different strands of the party as he announced which candidate would receive his nomination.
"I believe strongly that David Miliband is the right person to lead our party, first in Opposition and then back to power," Mr Darling wrote in a letter to constituents. "He will re-engage Labour with the public, particularly those with whom we have lost touch. David has always campaigned and argued positively for the Labour Party and that is the kind of leadership we need."
Mr Darling, who is serving as shadow chancellor but has announced his intention to step back from frontline politics when a new frontbench team is elected later this year, is set to appear alongside Mr Miliband at a campaign event in London today.