Jeremy Corbyn is preparing to attack David Cameron’s demand for an “emergency brake” on EU migration, claiming that the plan is discriminatory and unfair.

In a move that will alarm some pro-Europeans in his party, Mr Corbyn is planning to make an “alternative argument” . He will suggest that Mr Cameron has been “playing at the edges” in his renegotiation and will suggest a crackdown on the undercutting of wages by unscrupulous agencies that pay Eastern European workers in the UK below the minimum wage.

However his perceived lack of enthusiasm for any deal that Mr Cameron comes up with will alarm those in Labour who believe Mr Corbyn is still at heart a Eurosceptic, and will make little effort to drum up support for the In campaign.

It will also anger those on the right of the party who fear it could give the Tories and Ukip ammunition in vulnerable seats, as they would seek to portray Labour as out of touch on immigration.

A source suggested Mr Corbyn’s response to any deal Mr Cameron reaches in Brussels was likely to focus on companies using migrant labour to force down wages – which he will say Mr Cameron’s renegotiation has failed to address.

Some shadow cabinet members would prefer Mr Corbyn to endorse whatever deal Mr Cameron comes back with. On 14 February an open letter signed by Lord Kinnock, Hilary Benn, David Blunkett and Jack Straw said they were backing Mr Cameron.