The Church of Scotland's social care arm is employing hundreds of staff on controversial zero-hours contracts.

More than 500 Crossreach staff are currently on the terms - which guarantee neither work nor pay.

The Kirk last night defended their use, saying they applied to relief workers who cover sickness absence and holidays.

A spokesman added the Church itself did not employ anyone on a zero-hours contract.

Trade unions criticised the Kirk last night, calling on it to give workers "the contracts that they deserve".

It emerged this week up to one million people across the UK could be in zero-hours jobs - four times as many as previously thought.

Across Scotland, councils, care homes, and high-street names such as and McDonald's employ staff on the terms.

Stephen Boyd, from the STUC, said it was disappointing to discover the contracts were used by the Church of Scotland, which he said had in the past been a "particularly robust supporter of the case against the increasing insecurity of work".

He added: "We would encourage the Church of Scotland to look at this and, if it can, act to put these people on the contracts that they deserve".

Crossreach runs homes for the elderly across Scotland as well as supporting children, families, drug addicts, the homeless and those with mental-health problems and learning disabilities.

Unison Scotland said it was opposed to zero-hours contracts for a range of reasons, including the insecurity and stress for workers, as well as the low pay normally associated with the zero-hours jobs.

"We believe good social care employers should want to provide a quality care service, rather than one with a high staff turnover," a spokesman said. "This is best achieved by offering decent pay, terms and conditions."

Pat Rafferty, Scottish secretary of the Unite union, denounced the contracts as a disgrace that left hard-working people in limbo.

"They are widespread across the economy and not merely isolated in the retail sectors. It seems the more we scratch the surface of this scandal, the more companies and organisations become embroiled," he added.

Business leaders insist the contracts can provide workers with much- needed flexibility, but Business Secretary Vince Cable has ordered a review of their use.

The Herald revealed yesterday that his Cabinet colleague Iain Duncan Smith had warned too many people were on the controversial terms.

The former Tory leader called for far fewer zero-hours jobs and said more workers should be given "proper" contracts.

The Scottish Government has warned it could ban companies from winning lucrative public procurement contracts if they employ staff on zero-hours deals.

The Church of Scotland last night said it did not have an official position on zero-hour contracts.

However, it added it did not employ anyone on the terms and even those subcontracted to the church from agencies, such as security staff and cleaners at its main offices, are all on full-time contracts with their companies.

A spokesman for the Church said: "Crossreach, the Social Care Council of the Church of Scotland, currently employs 546 staff on relief contracts and has employed people on this basis for a number of years.

"Relief staff are used to cover periods of absence created by sickness, annual leave or vacancies.

"Relief staff form a bank of workers attached to a specific service and have the opportunity to accept or decline any shifts offered."