A Christian employed as a manager by a Christian charity was forced to discriminate against fellow employees because they were non-Christian, it was claimed today.

Mark Sheridan, an ex-manager of Prospects, a charity that supports adults with learning difficulties, told an employment tribunal that he had to tell non-Christian staff they were not eligible for promotion.

He resigned from the charity, which has its headquarters in Reading, and he and another former employee of Prospects are taking claims of constructive dismissal to an employment tribunal in Abergele, North Wales, in what has been described as potentially a landmark case.

The British Humanist Association and the trade union Unison are supporting the two claimants in the case, expected to last eight days.

The association said Mr Sheridan left his job after the charity changed its recruitment policy so it only hired practising Christians, and told non-Christian staff they were no longer eligible for promotion.

In his witness statement read out to the tribunal, Mr Sheridan claimed that his management job became increasingly difficult as the policy was implemented, and eventually he resigned.

He is also claiming that being forced to work within such a restrictive employment policy was highly detrimental to his mental and physical health.

The case is the first to test the extent to which religious organisations can attach “Genuine Occupational Requirements” as laid down in the Employment Equality (Religion or Belief) Regulations 2003, to their jobs.

Hanne Stinson, chief executive of the British Humanist Association, said: “We are pleased to be involved in what is potentially a landmark case in the area of discrimination on grounds of religion or belief. This is why this case is so important. We believe that since that law came in, some religious organisations are actually discriminating more in their employment practices, and this case appears to confirm that.”

Ms Stinson continued: “The exemptions in the equality legislation are there to allow for discrimination for very specific positions – clearly a Cardinal needs to be a Catholic. But we do not believe that the law was intended to allow religious, or indeed humanist, organisations to discriminate wholesale in their employment policies and practices.

“We believe that this kind of blanket discrimination is both unacceptable and, as the tribunal will hear, puts the quality of services at real risk. As this charity discovered, religious organisations that recruit from a restricted pool of candidates, in this case practising Christians, will find it difficult to employ appropriately qualified and experienced staff.”

The second claimant, who will argue that she was denied promotion on grounds of her non-religious beliefs, is being represented by Unison.