Alan Parker, chairman of Brunswick Group, and Martin Sorrell, CEO of WPP, are among more than 1,200 business leaders who signed a letter backing the campaign to keep the UK in the European Union.

Alan ParkerAlan Parker

"Our reasons are straightforward," the letter published this week says. "Businesses and their employees benefit massively from being able to trade inside the world’s largest single market without barriers. As business people, we always look to the future — and a future inside the EU is where we see more opportunities for investment, growth and new jobs."

UK voters go to the polls June 23 in a referendum to decide whether the UK remains in the EU or drops out. Polls predict the vote will be close.

Other notable business signers include Richard Branson of Virgin Group and Michael Bloomberg, the Bloomberg founder and former New York mayor who helped fund a campaign for the UK to remain in the EU.

Parker, who was knighted by Queen Elizabeth in 2014, founded Brunswick in 1987.