Labour would make it a criminal offence for employers to undercut pay and conditions by exploiting migrant workers, Ed Miliband has said in Norfolk today.

In the second of what Labour said were five key pledges for next year's general election, Mr Miliband said the move would form part of a package to deal with concerns over immigration, also including the introduction of 'proper' entry and exit checks, a limit on in-work benefits for EU migrants, and the extension to two years of the period before which immigrants can claim out-of-work benefits.

The pledge came as a leaked document showed that Labour activists had been urged by party chiefs to 'move the conversation on' when voters raise the issue of immigration.

The paper, setting out plans to counter the challenge from Ukip, was sent to dozens of MPs in constituencies where the eurosceptic party threatens to rob the opposition of the votes needed to return to power at next May's general election, the Daily Telegraph said.

Mr Miliband made no direct reference to the document during a speech on immigration in Great Yarmouth Town Hall.

But he insisted: 'When people worry about the real impact immigration has, this Labour Party will always respond to those concerns, not dismiss them.

'It isn't prejudiced to worry about immigration, it is understandable.'

And he said that leaving the EU, as Ukip advocates, would be 'a disaster for jobs, business and families here'.

Labour has already promised to take action to stop migrant labour undercutting home-grown workers, by increasing fines for paying below the national minimum wage, closing loopholes in agency worker laws, banning recruitment agencies from hiring only from abroad, and calling for a new law against extreme cases of exploitation.

Mr Miliband said he wants to end an 'epidemic of exploitation' which has led to 'truly shocking' examples of migrant workers having their wages withheld and being forced to live in appalling conditions.

'When people can be exploited for low wages or endangered at work, it drags the whole system down, undercutting the pay and conditions of local workers,' said the Labour leader.

'We must end the epidemic of exploitation. We must stop people's living standards being undermined by scandalous undercutting.'

Setting out his new proposals, he said: 'We are serving notice on employers who bring workers here under duress or on false terms and pay them significantly lower wages, with worse terms and conditions.

'We will make it a criminal offence to undercut pay or conditions by exploiting migrant workers.'

Aides said that bringing in foreign labour to undercut the pay and conditions of existing workers will not be sufficient, under Labour's plans, to secure a prosecution on its own, but it could be used as a piece of evidence of exploitation.

To secure a conviction, evidence will be needed that an abuse of power has occurred and that migrants were employed on significantly different terms to local workers.

Great Yarmouth is 73rd on Labour list of target seats. Brandon Lewis won the seat from Labour's Tony Wright at the last General Election in 2010.

But a poll by former Tory donor Lord Ashcroft in July found the party had not yet won back enough support to re-claim the seat. The survey of 1,000 voters in July put the party at third, behind the Conservatives and the UK Independence Party (UKIP).

UKIP has set its sights on Great Yarmouth in the Westminster elections next year after winning 10 out of 13 borough council seats in the May elections, and its strong showing in the 2013 county council elections.