Labour MPs Clive Lewis, David Lammy and Geraint Davies have joined trade union leaders in backing a new Labour campaign for free movement.

Their participation in the campaign underlines the tension between the Labour party's official line that free movement will end after Brexit, and the theory expounded by many leading Labour politicians that migration has forced down wages.

The MPs were joined by the MEP Lucy Anderson, Transport Salaried Staffs Association general secretary Manuel Cortes, National Executive Committee members Ann Black and Darren Williams, and the leaders of the Bakers, Food and Allied Workers Union.

Cortes said: "We are fully committed to challenging and fighting unscrupulous bosses who exploit EU migrants. We put the blame firmly on the perpetrators not the victims. We strongly believe that free movement enriches our society."

A statement from The Labour Campaign for Free Movement said: "Migrants are not to blame for falling wages, insecurity, bad housing and overstretched public services. These are the product of decades of underinvestment, deregulation, privatisation, and the harshest anti-union laws in Europe."

It argued that free movement was "the best way to protect and advance the interests of all workers", and warned: "Limiting migrants’ access to public services and benefits, will make it easier for unscrupulous employers to hyper-exploit migrant labour."

The campaign will attempt to intervene in the internal Labour immigration debate, with motions planned for the Labour conference in September.

Once viewed as a key ally of Jeremy Corbyn, Lewis resigned from the shadow cabinet in February in order to defy the party line and vote against triggering Article 50, which began the process of exiting the EU.

By contrast, Corbyn has become more outspoken on the negative impact of free movement, warning against "the wholesale importation of underpaid workers from central Europe in order to destroy conditions".

Lammy, the MP for Tottenham, was among the first to question the EU referendum result, with a call for parliament to overrule the vote.

Davies, the MP for Swansea West, has also backed a second referendum.