In March last year, the commission released a draft code of practice on 'monitoring and personal data privacy' at work, which proposed employers be held liable for criminal prosecution if they spy on staff without first having evidence of 'criminal activity or serious wrongdoing'.

But after three months of consultation, Mr Tang said, it emerged that using a 'best practice guideline' was the preferred option instead.

He expressed 'regret and disappointment' over what he said was the trade unions' failure to participate in the consultation.

'None of the registered trade unions gave us any written submission. We had replies from public organisations, private companies, professional bodies and employer organisations, but nothing from trade unions.'

Li Fung-ying, a lawmaker from the Federation of Hong Kong and Kowloon Labour Unions, disputed Mr Tang's criticisms.

'They came to the Legco [home affairs] panel and we gave them our feedback,' she said. 'I don't understand why he said trade unions gave no response.'

Mr Tang said that when the commissioner went to the Legco panel meeting to discuss the issue last April, the federation even opposed the idea of making any violation a criminal offence.

But Ms Li defended her objection to a binding code, saying: 'There wasn't a definition to what workplace was about.

'We think this will be very dangerous to many working parents as they have to hire domestic helpers to look after their kids. They will be subject to criminal liability if they have to check on their maids,' she said.

Under existing laws, there is nothing to stop employers monitoring the content of e-mails and telephone calls that are paid for by the company.