Any walkout could involve more than 300 workers

Some staff at Birmingham Heartlands and Solihull NHS Trust told the union last month that they wanted a ballot.

Unison said contract cleaners and porters get paid less than their in-house counterparts.

It gave their employers, Initial Hospital Services, until 15 October to table a pay increase.

Industrial action

The hospital trust has one of the worst rates of MRSA infection in England.

A Unison spokesman said: "Whether there is a result on Friday or not, we will not be releasing any details until Monday at least."

Speaking last month Ray Salmon, regional organiser for the union in the West Midlands, said: "If there's nothing on the table on 15 October then we could be pursuing industrial action by mid to late October."

Figures compiled by Unison show contract cleaners working at the two hospitals in east Birmingham and Solihull currently earn �4.65 an hour, while porters take home �4.95.

Cleaners and porters employed directly by the trust earn �4.95 and �5.20 an hour respectively with sick pay, pension provision and other benefits.

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

Department of Health figures published last month showed Birmingham Heartlands and Solihull NHS Trust had the seventh worst rate of MRSA infection among general acute hospitals in England over the past three years.