The furore over the Labour leadership battle gathered pace after David Miliband apparently criticised his brother Ed's maiden speech.
The shadow foreign secretary was in the hall in Manchester to watch the address, praising it afterwards as "very strong" and "nerveless".
But ITV News claimed to have caught him on tape making a barbed comment to the party's deputy leader, Harriet Harman.
Following a key passage where Ed condemned the invasion of Iraq as "wrong", his elder brother was said to have turned to Mrs Harman sitting next to him and said: "You voted for it, why are you clapping?"
She replied: "I am clapping because as you know, I am supporting him."
Mr Miliband now has until 5pm on Wednesday to decide whether to put himself forward for election to the shadow cabinet.
