He told party delegates in Dundee at the weekend that Labour would win a “reasonable majority” if a snap election were held to resolve the Brexit turmoil and revealed he is already preparing a budget.
Labour said it would create tens of thousands of jobs in Scotland’s offshore wind industry by fully seizing the potential of the sector.
The industry in Scotland has suffered a body blow recently as two BiFab yards in Fife lost out on lucrative contracts for the fabrication of turbine jackets and floating platforms from the Moray East and Kincardine offshore wind farms to firms in the United Arab Emirates, Belgium and Spain.
But Mr McDonnell insisted firms like Bi-Fab would flourish under a “real industrial strategy” which would be a flagship pledge of Labour’s next elelction manifesto.
“Under Labour, Scotland will be at the heart of a green industrial revolution in our energy sector that could save this planet,” he said.
“You know that already 60 per cent of the UK’s onshore wind capacity is in Scotland.
“Labour has developed ambitious plans for expanding onshore wind. At least 60 per cent of that new capacity will be here and could mean 20,000 new jobs in Scotland.
“With another 42 gigawatts of capacity under Labour from offshore wind, that could be another 15,000 jobs in Scotland. And when we roll out our UK-wide home retrofitting programme that could be close to 15,000 more jobs here in Scotland.
“That’s a total of another 50,000 new, well-paid, unionised jobs as a result of our green industrial revolution.”
Labour has been trailing the Conservatives in the polls but Mr McDonnell said that lead could be overturned in a snap election.
“We need another two to three points to take us into government and that is what we will get when we go into that campaign,” he told a fringe event in Dundee.
Mr McDonnell said that he and party leader Jeremy Corbyn were already preparing for government.
While he accepted it was “almost impossible” for Labour to force a general election under the current system of fixed term parliaments, Mr McDonnell said: “Just in case there is an election, we are preparing for an election, we’ve got all the campaign plans ready. We’ve been testing messaging, we’ve got the grid ready to go. If there is a general election we go straight into election mode.”
He added: “I’m going through a first budget exercise, so I’m sitting down with all the shadow cabinet teams to talk about their plans, costing them and then look at priorities.
“We’re in that situation where literally we’re preparing for government.”