Britain will have a stronger voice in the world if it stays in the European Union, the opposition Labour Party will say on Monday as it seeks to persuade its voters to back an "In" vote before an increasingly close-run June 23 referendum.

Polls show Britons are evenly divided over how to vote next week, prompting Conservative Prime Minister David Cameron to take a back seat to allow leading Labour figures to appeal to undecided left-leaning voters.

With Conservative supporters deeply divided over the EU, Labour voters will be crucial to winning the referendum.

While the party officially backs In, its leader Jeremy Corbyn has been criticised for not doing enough to support the campaign. Corbyn has rejected the charges.

Former Labour prime minister Gordon Brown, credited with making a decisive last-minute intervention in a Scottish independence referendum two years ago, will launch what the In campaign described as a "Labour fight-back" on Monday.

Scots voted 55 percent to 45 percent in 2014 to maintain their 308 year-old union with the England.

"Voting to Remain is about a positive, stronger future and is the alternative to a Tory (Conservative) Brexit. It is stronger for jobs, for rights at work and maintaining a British voice on the world stage."

Brown will say that Britain should be "leading not leaving" the EU and push for more action to protect workers' rights, create jobs and provide more support for communities which are under pressure from high rates of migration.

In a separate speech, Labour's foreign affairs spokesman, Hilary Benn, will say Britain's national interests are best served by being part of international institutions and alliances that give it influence in the world.

"We have now reached the defining moment in this referendum. The Brexit train is threatening to pull away from the station ... taking us down a track that can only lead to disaster," he said before making the speech.

"That's why it is so important that we make the patriotic case for remaining in the EU and keeping Britain great."