"The best champions, the best advocates of manufacturing and the vehicle industry are the three people standing in front of you who will stand up and fight for the auto industry and manufacturing in the House of Commons, " said Brown, who led Labour and the country from 2007 to 2010.
The Labour Party has promised to repeal the Lobbying Act, according to the party's leaked manifesto.
The speech coincided with the leak of a draft manifesto showing Corbyn, a lifelong socialist, will fight the election on a markedly left-wing platform, planning to renationalise industries and boost public spending.
Video Corbyn denies using 'Trumpian' rhetoric
"We have just unanimously agreed the contents of it".
"We've amended the draft document that was put forward in the most informed, interesting, sensible discussion and debate in our party".
"I think we are going to see a manifesto which is progressive and for the people of this county".More news: Kohl's Corporation (NYSE:KSS) Issued With 'Reduce' Rating At Gordon Haskett
The Labour leader refused to take any questions from the waiting press-pack.
He also said the Conservatives' record on the NHS "shames" Britain, adding that Mrs May risks "leaving the country more socially divided than ever unless there is a change in policy".
Labour election coordinator Andrew Gwynne said the leaked document was not the final manifesto but "a draft of policy ideas".More news: Local election results in Scotland likely to heal COSLA rift
The 43-page document, which includes plans to nationalise key industries and reverse years of austerity, has been denounced by the Conservatives as a recipe for taxes and borrowing that would leave the United Kingdom on the "road to ruin".
In the draft manifesto, Labour said it would reform corporate governance, rein in "boardroom excess" by setting a 20:1 limit on the ratio between the lowest and highest paid in companies with government contracts and protect small businesses.
It also said it would not accept a "no deal" from the European Union, criticising May's stance that "no deal was better than a bad deal" at the end of Brexit negotiations.More news: Factors to Note Before Buying Dean Foods Company (DF)
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