Labour has ignited a row with supermarket giant Tesco by planning to attack the chain for employing foreign workers.
Shadow immigration minister Chris Bryant is due to condemn Tesco and other leading companies for being "unscrupulous" and pursuing cheap labour from Eastern Europe.
In a speech on Monday, he is expected to claim Tesco moved its distribution centre to Kent where a "large percentage" of the staff are from the eastern bloc.
The firm has already hit back, denying having a distribution centre in the county and insisting it does not actively seek to recruit abroad.
A spokesman said: "It is wrong to accuse Tesco of this. We work incredibly hard to recruit from the local area and have just recruited 350 local people to work in our Dagenham site."
The company is also understood to be writing to Labour to complain about the "unfair" slight.
Controversial: Frontbencher Chris Bryant
Mr Bryant's speech appears to be the Opposition's latest attempt to wrestle back the initiative on immigration, which is a top concern for British voters.
The party is also battling to get on the front foot after a quiet summer which has seen their poll lead slip and Ed Miliband face internal criticism for failing to set the agenda.
Mr Bryant will admit immigration can have a negative effect on labour markets, according to extracts from the draft speech reported in The Sunday Telegraph.
"The biggest complaint I have heard, from migrants and settled communities alike, is about the negative effects migration can have on the UK labour market. And I agree," he is due to say.
"It is unfair that unscrupulous employers whose only interest seems to be finding labour as cheaply as possible will recruit workers in large numbers in low-wage countries in the EU, bring them to the UK, charge the costs of their travel and their substandard accommodation against their wages and still not even meet the national minimum wage.
"That is unfair. It exploits migrant workers and it makes it impossible for settled workers with mortgages and a family to support at British prices to compete."
He will claim Tesco had recently decided to move its distribution centre in Kent but slashed pay for existing staff, penalising Britons.
"The new centre is larger and employs more people, but the staff at the original site, most of them British, were told that they could only move to the new centre if they took a cut in pay," he will say.
"The result? A large percentage of the staff at the new centre are from [the] Eastern bloc."
As well as highlighting the recruitment of hundreds of local staff, Tesco made clear that the distribution centre is in the east London borough of Barking and Dagenham and was formerly in Essex, not Kent.
Next is also criticised for allegedly seeking out Polish staff
Mr Bryant's address is also set to target high street chain Next, who he accused of printing leaflets in Polish to attract workers from the country.
Extracts from his speech say: "Look at Next Plc, who last year brought 500 Polish workers to work in their South Elmsall warehouse for their summer sale and another 300 this summer.
"They were recruited in Poland and charged £50 to find them accommodation. The advantage to Next? They get to avoid Agency Workers Regulations, which apply after a candidate has been employed for over 12 weeks, so Polish temps end up considerably cheaper than the local workforce, which includes many former Next employees."
The MP's speech will make clear that neither firm has broken the law.
A spokesman for Next told The Sunday Telegraph: "Without access to the facts, it is difficult to comment on what Mr Bryant is claiming. On the face of it, his allegations seem unlikely."
Tesco, which is the third largest retailer in the world, has 3,146 stores in the UK and employs more than 300,000 staff here.
Next has more than 500 shops across the country, employing around 54,000 people.
Tory MP for Harlow Robert Halfon attacked Labour for allowing "uncontrolled immigration" during its time in office, but also joined the row against Tesco.
On Sky News, he accused the supermarket chain of behaving "disgracefully" towards staff when it closed its distribution centre in his constituency.
"They said they were building a new Dagenham plant and the Harlow plant in my own constituency would be alongside it," he said.
"The moment the plant was built it was suddenly announced the Harlow plant would close. They then said to the British Harlow workers yes, they could have jobs in Dagenham, but it would be at lower pay after transitional costs had been taken in.
"As a result the majority of them couldn't afford to work in Dagenham and had to take redundancy. Tesco in my view behaved quite disgracefully and quite ruthlessly."