'I would feel uncomfortable signing something that discriminates against my colleagues,' he said.

Mr Green has completed stints as acting principal at the Sha Tin senior secondary college, which is one of 10 United World Colleges around the world that pride themselves on promoting international understanding and being secular, liberal and progressive.

He said he had decided to take a moral stance on the issue and plans to move to New Zealand after his wife fulfils her teaching commitments next year.

Mr Green said the partners of married staff received certain benefits such as medical cover and return airfares.

'I have a number of gay colleagues. None of them are thinking of getting married at the moment but to me that isn't the point. The point is we shouldn't be introducing discrimination,' he said.

'I've reached the point ... where I can perhaps afford to make these decisions more than my younger colleagues. Whatever their sexual orientation it shouldn't matter in regards to the recognition of marriage.'

Mr Green said he had received much support from teachers and students after he published his views in the school's student magazine. 'I'm not a lone voice on the staff,' he said.

Students who contacted Education Post supported Mr Green's actions and are starting a petition that will be sent to the UWC international office. Alex Chadwick, 17, said he was 'shocked and outraged'.

'I honestly could not believe that the board at a college that held its own values in such high esteem could be so homophobic and discriminatory,' he said. 'I think John Green did absolutely the right thing by refusing to sign. It isn't an issue of money or of favouritism. It's an issue of principle.'

The clause regarding marriage was changed last year. However, staff are on two-year contracts and this was the first time Mr Green has had to sign the new contract.

The contract defines marriage for those married outside of Hong Kong as 'between a man and a woman celebrated or contracted outside Hong Kong in accordance with the law in force at the time and in the place where the marriage was performed'.

When Mr Green contacted the school's board, he said he was told the definition was changed so that it was in keeping with Hong Kong law and reflected 'local values'. 'Even if it was a local value you could say we have our own set of values,' he said, adding that it went against the basic tenets of the UWC movement.

He also raised the issue with the school's international board, which controls all UWC colleges, but was told it was a contractual issue between staff and the college.

'The international board should take action on this. It brings the whole of the UWC movement into disrepute,' he said.

Hong Kong Human Rights Monitor director Law Yuk-kai said the move could make the school a less attractive employer for some teachers. However, he said there was no legal recourse for teachers to challenge the rule because same-sex marriage was not officially recognised under Hong Kong law.

In his article, Mr Green wrote that the contracts also included clauses referring to confidentiality, 'which seemed to be totally 'over the top' and more appropriate to a highly secretive commercial enterprise than an educational institution'. He cites a clause that states the appointee must not communicate with the media on any matter relating to the college without written consent of the principal or supervisor.

Mr Green said the clause may have been prompted by the negative publicity the college received in 2006 when a teacher was fired despite being cleared of allegations of misconduct.

Li Po Chun principal Stephen Codrington said the contracts had been updated to comply with Hong Kong law and all staff except Mr Green had signed. 'It wasn't a definition in order to make a stand on the matter of marriage or anything like that. It was to make sure the contract was consistent with the law of Hong Kong,' Dr Codrington said.

However, Mr Green noted that part of the Hong Kong legal definition of marriage - 'to the exclusion of all others' - had been omitted. Dr Codrington said it was a 'theoretical discussion ... we don't have any teachers in same-sex marriages on staff.'

He understood Mr Green was concerned with the principle of the matter but said: 'I think to some extent John might be seeing the overall movement as being a bit more liberal than it is.'

Dr Codrington said the school's practices were identical to other UWC colleges, which were all required to conform to local laws. 'What we're doing is exactly in line with the other colleges,' he said.

Regarding the confidentiality clause, Dr Codrington said the college's lawyer had said the clause was included in about 85 per cent of Hong Kong employment contracts.