By Kylie MacLellan

LONDON (Reuters) - The north of England would be hit hardest by a vote for Brexit, the campaign to keep Britain in the European Union will say on Thursday, as it steps up its push to win over opposition Labour supporters seen as key to securing an 'In' vote.

With opinion polls showing momentum swinging towards 'Out' ahead of next week's referendum, the In camp has embarked on an appeal to Labour voters it fears have been turned off by media coverage dominated by splits within Prime Minister David Cameron's Conservatives over the EU.

England's north, a traditional Labour stronghold where many working-class voters have been hit by the demise of much of the region's coal and steel industries, has seen a rise in support for the anti-EU UK Independence Party.

Presenting research by the In campaign that says 10 percent of jobs, more than 700,000, in the north are linked to EU exports, former Labour prime minister Gordon Brown will say European Union funds have helped regenerate the region.

Many in the north blame former Conservative leader Margaret Thatcher, in office from 1979 to 1990, for the collapse of industry in the region.

"In the 1980s the Tories (Conservatives) turned our industrial heartlands into industrial wastelands ... What stood between our communities and further devastation was the European structural funds, regional funds and social funds," Brown will say at a speech in Manchester.

Jobs in key sectors, such as car manufacturing, will be at risk if Britain leaves the bloc and high tariffs are imposed on exports, he will say.

"I ask everyone who works in the car industry, the aviation industry, the pharmaceutical industry and the food and drink industry in the North, to tell their fellow citizens of the importance they attach to the jobs that come from their exports to Europe and the risks of losing these jobs to Brexit."

The north of England has also been allocated 2.1 billion pounds ($3 billion) of EU funds to support infrastructure, higher education, agriculture and small business between 2014 and 2020 which would be lost if Britain votes to leave the bloc on June 23, the In campaign will say.

(Editing by Stephen Addison)