The reports came as Mr Corbyn’s rival Yvette Cooper issued a stark warning over the party’s potential lurch to the left, claiming that, like in the 1980s, it would be used by the Tories to push through much more right-wing policies.
Speaking to Huffington Post, Ms Cooper, who is backed by former prime minister Gordon Brown, also said she could still pull off a surprise victory against Mr Corbyn.
And she confirmed she would not serve under Mr Corbyn in his shadow cabinet if he wins.
Blairite leadership rival Liz Kendall, who also said she would refuse to serve under a Corbyn leadership, issued a stark warning yesterday that Labour faced being “wiped out” if it moved dramatically to the left.
According to reports, Mr Miliband, who led the party to a dismal defeat in May which saw them lose 40 of their 41 Scottish seats, will also decline to serve under Mr Corbyn.
It is understood Mr Miliband has told friends he “wants a break” from national politics but also wants to focus on campaigning against inequality.
However, the report has been denied by one source who insisted it was just “speculation”.
Mr Corbyn’s inner circle had hoped that if the former Labour leader joined the shadow cabinet it would provide some assurance for those concerned by a potential lurch to the left.
With the result due next Saturday, Ms Cooper has enjoyed a late surge in support after leading the attack on the Conservative government’s slow response to the Syrian refugee crisis.
Speaking yesterday, she warned of the damage caused by a left-wing leadership of the Labour Party, evoking the memory of Margaret Thatcher.
She said: “What happened in the 1980s made it possible for the Tories to become more right-wing, made it possible for Margaret Thatcher to take a harder line because the Labour party wasn’t as a credible threat, a credible alternative. It’s about making it harder for us to defeat them. It’s divided parties, people don’t want a party fighting among itself.”
And she also warned that people should not bank on a Corbyn victory or one by his main rival, shadow health secretary Andy Burnham.
She said: “I think there’s a huge number of people who haven’t voted yet. Some of that is delayed ballots, some of that is people being on holiday, some of that is people wanting to take time to get it right,”
“And I think that is a good thing. It also fits with what I’m saying, which is we’ve got to get this right because this is really important. The whole point of the Labour Party when we were first founded was to put principles into practice.
“If we don’t do that, we stop being the Labour Party, we just become a protest movement.”
She also appealed to the party not to embrace Mr Corbyn’s anti-military pacifism.
She said: “You always have to work for peace, but sometimes you have to fight for justice.
“And sometimes you have to be prepared to. We were absolutely right to intervene in Kosovo. I also still think it was the right thing to do to stop what was happening in Benghazi [in Libya] and the huge humanitarian crisis that there would have been, even though the follow-on consequences have been really difficult. There’s a big challenge about what should have been done in the aftermath.”