BILLIONAIRE tycoon MIKE Ashley has been revealed as the biggest employer of zero hours contracts, that potentially stop thousands of his staff from getting a second job.

The sportswear retailer, owned by the Newcastle United owner, employs about 20,000 workers on the soon-to-be-outlawed terms, which offer no guarantee of working hours from week to week.

A copy of a 2012 contract told an employee: “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 Direct.com Retail Limited. You would then be prevented from taking it up.”

The revelation comes a year after senior management at Sports Direct took share option bonuses worth three times their pay.

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. Zero hours contracts are widespread in the retail and fast food industries.

Critics say the contracts 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.

Sports Direct declined to comment.