Jeremy Corbyn claims Yorkshire supermarket giant Asda is one of the UK's worst employers for treatment of its staff

The party promised to take on what it describes as “bad bosses who have exploited, ripped off and dehumanised workers”, while singling out Asda as one of the worst offenders.

But the firm said in a statement today: “We entirely reject these claims about our contract and employment status, which are absolutely at odds with both how we operate our business or the regard in which we hold our colleagues.”

Sports Direct, outsourcing giant ISS and Uber were also criticised ahead of the publication of a ‘work manifesto’ containing a host of new measures to improve workers’ conditions.

Labour went on the offensive as Mr Corbyn was to meet cleaners and catering staff who have organised themselves against employers in London today.

Asda has been embroiled in a long-running dispute with union bosses, who say staff were threatened with the sack if they refused to sign up to a new contract.

The deal increases basic hourly pay but ends paid breaks, cuts premium pay on most bank holidays and reduces the number of hours rated as better-paid night shifts.

The firm says the changes, which prompted a petition signed by 23,000 people, represent an £80m investment in pay for over 100,000 colleagues and ensures the same terms and conditions apply, which is fairer to staff.

Leeds-based Asda, the UK’s third largest supermarket, saw its profits rise by 13 per cent to £805, while payouts to directors are reported to have increased to £12m from £9.5m the year before.

In its work manifesto today, Labour is pledging a £10 minimum wage for all workers, a ban on zero-hours contracts and a requirement that all mid-shift breaks are paid.

Though the commitments have already been announced, Labour was highlighting them as part of its work manifesto.

Labour would also ban “bogus” self-employment, so bosses cannot evade workers’ rights, and repeal anti-trade union legislation, if it wins the December 12 election.

Mr Corbyn said: “The Conservatives are on the side of bad bosses who have exploited, ripped off and dehumanised workers.

“We’ll call time on insecure and unsafe work that leaves people without the rights and dignity they deserve. We’ll call time on discrimination in the workplace that leaves women vulnerable to harassment and unequal pay.

“And we’ll call time on the running down of workers’ rights to organise collectively to boost their pay and improve their working conditions.”

In its statement, Asda said: “Despite the huge pressures facing our sector, we have worked to give a pay increase to almost 120,000 of our retail colleagues in return for a degree of flexibility that is standard in our industry and ensures fairness for all our colleagues.

“Our contracts include a market leading benefits package - which offers a colleague bonus and Sharesave scheme - and we do not use zero hour contracts.”

Uber defended its record, saying “drivers are at the heart of our service”. And ISS insisted its employees are paid “fairly and competitively” with a “real living wage” since 2012.

An Amazon spokesman said Labour's claims about it were "false and, despite sharing the facts with the Labour party on numerous occasions, they’ve chosen to ignore them".