The document suggests Labour will commit to nationalising the railways, creating a public energy provider, rolling out collective bargaining for employees, and adding £250bn of public spending.
Mr Corbyn cancelled an appearance at a poster launch this morning to deal with “internal matters”.
Labour’s shadow cabinet and party leadership assembling in London for a scheduled meeting to sign off the manifesto, were quizzed on its contents by reporters despite not having seen the document before.
The leaked draft indicates Labour will commit to nationalising bus firms and the Royal Mail, and scrap university tuition fees in England.
It also suggests Labour will refuse to set a target on cutting immigration and rule out leaving the European Union without a deal.
On Scotland, the draft manifesto commits the party to opposing a second independence referendum, extending the High Speed 2 rail line to Scotland, and creating a Scottish National Bank to encourage infrastructure investment.
It also confirms Labour’s support for a Constitutional Convention to create a federal UK, and says the party would appoint a Minister for England within the UK Department for Local Government to work with the Scottish, Welsh and Northern Irish secretaries.
In an effort to bridge party divides over Trident, the manifesto commits Labour to the nuclear deterrent, but in a nod to Mr Corbyn’s opposition to the weapons it states “any prime minister should be extremely cautious about ordering the use of weapons of mass destruction which would result in the indiscriminate killing of millions of innocent civilians”.
On pensions, the draft says no increase in the retirement age beyond 66 will go ahead.
To pay for the policy pledges, Labour has already announced plans to hike corporation tax to 26% by 2022, bringing in an extra £20 billion for the Exchequer, and indicated that people earning more than £80,000 will face tax rises.
But the manifesto indicates a further levy on firms “with high numbers of staff on very high pay”.
A spokesman for Mr Corbyn said: “We do not comment on leaks. We will announce our policies in our manifesto, which is our plan to transform Britain for the many, not the few.”
Recriminations over how the manifesto was leaked have already begun, with sources close to Mr Corbyn blaming Labour Party HQ for seeking to sabotage his policies, while others claimed the leader’s circle leaked the document themselves to increase media coverage and make it difficult for the pledges to be watered down.
Labour elections co-ordinator Ian Lavery told reporters at the poster launch: “Listen, you’ve done me a real favour because what I’ve seen this morning is that lots of people are terribly excited.”
He described the manifesto as “fantastic”, with “visionary policies”.
A Conservative spokesman said: “This is a total shambles. Jeremy Corbyn’s plans to unleash chaos on Britain have been revealed.
“The commitments in this dossier will rack up tens of billions of extra borrowing for our families and will put Brexit negotiations at risk. Jobs will be lost, families will be hit and our economic security damaged for a generation if Jeremy Corbyn and the coalition of chaos are ever let anywhere near the keys to Downing Street.”
The SNP candidate in Edinburgh West, Tommy Sheppard said Labour was in “chaos”.
He said: “The very fact that this draft manifesto has been leaked shows how divided and chaotic the Labour party are - most of their MPs do not even support these policies.
“In particular, Labour have broken every manifesto promise they have made on tuition fees, so no one will believe a word they say now.”
The document, which was due to be unveiled on Monday, must be approved by around 80 Labour figures, a senior party source said.
They include the shadow cabinet, the national executive committee, the parliamentary committee of the PLP, Welsh and Scottish Labour leaders, members of the national policy forum and trade union representatives.
Kezia Dugdale is not attending today’s meeting because of the clash with First Minister’s Questions at Holyrood, but a Scottish Labour spokesman said she had “made representations” on the manifesto’s contents.
Many of the more eye-catching proposals appeared in the 2016 Scottish Labour manifesto for the Holyrood elections.