From wiki on Corbyn and the EU.

...in September 2015, Corbyn said that Labour would campaign for Britain to stay in the EU regardless of the result of Cameron's negotiations, and instead pledged "to reverse any changes" if Cameron reduced the rights of workers or citizens... He also believed that Britain should play a crucial role in Europe by making demands about working arrangements across the continent, the levels of corporation taxation and in forming an agreement on environmental regulation...

...Corbyn made a speech on April 14, 2016, in which he supported staying in the EU...He summarised his stance as being "for 'Remain – and Reform' in Europe....

...In June 2016, in the run-up to the EU referendum, Corbyn said that there was an "overwhelming case" for staying in the EU. In a speech in London, Corbyn said "We, the Labour Party, are overwhelmingly for staying in, because we believe the European Union has brought investment, jobs and protection for workers, consumers and the environment".