David Miliband moved closer to bowing out of frontline politics after being filmed questioning his brother's position on Iraq during this afternoon's leader's speech.
The defeated leadership contender refused to applaud his brother Ed Miliband as Labour's new leader condemned the party's policy on the 2003 invasion.
"I do believe that we were wrong. Wrong to take Britain to war and we need to be honest about that," Ed Miliband told the Labour party conference, to applause.
"Wrong because that war was not a last resort, because we did not build sufficient alliances and because we undermined the United Nations. America has drawn a line under Iraq and so must we."
Footage from ITV News showed David Miliband turning to the party's deputy leader, Harriet Harman, and confronting her about her support for the U-turn.
"Why are you clapping? You voted for it," David Miliband is seen to ask Ms Harman.
She replies: "I'm clapping because he's leader and I'm supporting him."
Labour's party conference in Manchester continues to be dominated by David Miliband's uncertain future - but speculation is mounting that the shadow foreign secretary will rule out serving underneath his younger brother.
David Miliband has consistently denied he is leading a 'psychodrama' between the two brothers.
But his refusal to confirm whether he will stand in elections for the shadow Cabinet has placed his future career at the centre of discussions at the conference.
His partner Louise Shackleton was seen in tears behind the conference stage yesterday as Mr Miliband delivered his speech to Labour delegates on foreign policy.
She is said to have been furious at the nature of Ed Miliband's victory, which heavily relied on union votes. David Miliband told delegates: "I'll be fine."
He remains tight-lipped about his intentions, but must reveal his plans by Wednesday evening when nominations for the shadow Cabinet election close.
A role at the International Monetary Fund - possibly its top job - is one alternative under consideration, according to reports.