It is thought that all right-wing members of the NEC voted to keep him off, along with shadow cabinet member Jonathan Ashworth and at least two of the 12 trade union reps on the committee.
Outsiders can sign up to vote as "registered supporters" during a two-day window next week, but it will cost them £25 each.
Corbyn won almost 60 percent of first round votes when elected previous year.
The Labour MP Owen Smith announced yesterday that he will run to oust Jeremy Corbyn in the leadership contest, declaring that the party needs a "radical and credible" leader.
Those earning more than the minimum wage would pay just over £14-a-month.
"See, they can be forced back when we take decisive, immediate action", he said.
But in a rare move, Labour chair Paddy Lillis ruled that a secret ballot should take place. A source said over 2,000 people had joined general union Unite in less than 24 hours.
Mr Corbyn's close ally and Shadow Chancellor John McDonnell said: "Jeremy on the ballot".
The Labour leader's claim that he should be on the ballot automatically was passed by a margin of just four votes - including his own - in a fraught meeting of the party's National Executive Committee (NEC).
The NEC chose to raise the cost of joining as a registered supporter from £3 to £25 in a bid to prevent a repeat.
Some argued that was unlikely following the collapse in his support among MPs in recent weeks.
The intervention appears to be an effort by Unite to replicate last year's leadership campaign, when Mr Corbyn signed up tens of thousands of activists as registered supporters.
There was a dispute over the interpretation of Labour's rules, and whether they allowed Mr Corbyn to automatically defend his leadership, or whether he would also be required to secure 51 nominations.
Mr Foster said three contrasting pieces of advice had been given by three different lawyers, and the matter must be considered by a neutral court of law.
Pontypridd MP Mr Smith told BBC Radio 4's Today programme that it was right that Mr Corbyn took part in the race.
They also have to confirm that they support Labour's "aims and values".
He said: "I will stand in this election and I will do the decent thing and fight Jeremy Corbyn on the issues, just as he will do with me, and at the end of that I will stand behind whoever the leader is".
"But I hope and I expect it will be me".
But he said that he oppose attempts to hold Tony Blair in contempt over his decision to go to war in Iraq.
The other candidate, Angela Eagle also launched a call for the campaign to be free from abuse. Police said on Tuesday that Eagle's constituency office in northern England had been vandalised.
Mr Opperman said: "Whatever your views of Corbyn, the behaviour of Momentum and the extremists who are threatening MPs on a repeat basis is utterly unacceptable".