By Kylie MacLellan and William James
LONDON (Reuters) - Britain's main opposition leader, Jeremy Corbyn, appealed to leftist voters on Thursday to back "In" at a referendum on EU membership this month, defending immigration and rejecting criticism that he is not campaigning hard enough.
Strategists of the In campaign say Corbyn, elected leader of the Labour Party last year on a wave of enthusiasm for change, may help convince thousands of undecided voters in the north of Britain to back remaining in the EU and persuade younger supporters to vote at the referendum on June 23.
Those voters could break a deadlock in polls which suggest that Britons are largely split over which way to vote, with some signs of a shift toward leaving the bloc, fueled by days of headlines warning of near-record high levels of immigration.
Corbyn, who voted "No" to the EU's predecessor, the European Economic Community, in a 1975 referendum, said the blame for problems caused by immigration lay not with the EU, but with the Conservative government and employers.
"Let's not turn this into blaming people who travel, work and migrate around Europe," he told Labour members and supporters.
"Let's instead ensure governments respond to the needs of all communities and that unscrupulous employers that are grossly exploiting migrant workers and trying to limit their rights need to be dealt with."
The left-leaning Labour Party is seen as having a major role in keeping Britain in the EU because of its ability to reach voters who traditionally distrust Prime Minister David Cameron, leader of the official In campaign - such as younger Britons and those living in traditionally working-class areas.
Corbyn distanced himself from the In campaign, dismissing some of its arguments on the economic risks of Brexit as "hype", "myth-making" and "prophecies of doom".
Before the speech, Corbyn was criticized by one of his party's financial backers for not throwing his weight behind the campaign and failing to connect with Labour voters.
"He probably could do a bit more, let's hope he will," Tim Roache, head of the GMB trade union, told BBC radio.
"My biggest concern is that Labour voters will stay at home ... the reality is that the more people stay at home, the more likely it is that we will leave the EU."
Roache said he feared Labour supporters would not vote because they saw the referendum as a proxy for an internal battle within Cameron's Conservatives, which is deeply divided over the EU.
Corbyn rejected that criticism, saying he was getting the message out "as loudly as we can".
"The message will be very, very clear that we are campaigning to defend and extend workers rights and trade union rights in Britain," he said.
(Additional reporting by Michael Holden, editing by Elizabeth Piper)