Sports Direct has been accused of using zero hour contracts that demand exclusivity of employment from staff hired on them, an employment arrangement that could be banned.

The sportswear retailer, owned by billionaire Mike Ashley, employs around 20,000 workers on controversial zero hour contracts, which offer no guarantee of working hours from week to week.

According to the Independent, a leaked copy of a 2012 employment contract from Sports Direct says: "If you wish to undertake any work outside the company, whether paid or unpaid, you should raise the matter with your manager … it may be decided that the additional work would conflict with your duties at Sports Retail Limited. You would then be prevented from taking it up."

A government review of zero hour contracts is underway. Business secretary Vince Cable has been criticised for backing away from a complete ban, saying that many workers find their flexibility useful.

But he backs the idea of barring exclusivity clauses, which prevent staff working for other employers while on a zero hours contract, because they are unfair.

A private members' bill including this reform, submitted by Labour Ian Mearns MP, is currently working its way through parliament.

Office for National Statistics (ONS) data suggests there are around 1.4 million people on zero hour contracts in the UK, a figure disputed by the trade union Unite which puts it at a much higher 5.5 million.

Critics of zero hours contracts say they leave people in precarious financial situations because they do not have a secure number of hours or pay each week.

It is argued that intense competition for jobs in the labour market is working to some employers' favour, because they can avoid paying for the benefits permanent staff enjoy by forcing employees onto zero hours arrangements.

But proponents say zero hour contracts suit many people, including those who need to fit work around childcare and others who work seasonally.

They say this flexibility in the labour market is helping to keep people in work rather than unemployment, as is happening across regulation-burdened continental Europe.

Sports Direct declined to comment.