Any walkout could involve more than 300 workers, including contract catering staff, whom the union claims are similarly underpaid.

BBC: NEWS | UK | England | West Midlands | Cleaners strike ballot vote due

Some schools organise their own catering, while others contract the service in through private companies or the local authority.

BBC: School caterers 'confident' about meat supplies

One, to contract out airline catering supplies, is easy to resolve.

ECONOMIST: British Airways

"CAFRE has a service contract with Eurest, a subsidiary of Compass, which commenced in August 2011, to provide catering across the three CAFRE campuses, " she said.

BBC: Northern Ireland