Those who fail to comply will be warned by the Housing Authority, and could have their lease terminated. The authority will do random checks on the questionnaires to ensure their accuracy.

The data collected will be used to formulate the new income index among tenants, which will be the key in determining future rent levels.

But a public housing group and a lawmaker have said the survey will inconvenience tenants.

'This survey means all 600,000 tenants have to keep their income records for the study and future cross-checks.

'There are so many tenants with unstable incomes that it will be so troublesome for them to reply to the questionnaire,' said Ho Hei-wah, director of the Society for Community Organisation.

Lee Wing-tat, chairman of Legco's housing panel, said: 'There are many families where the children would not let their parents know how much they are earning.

'This survey could cause trouble among families.'

But a Housing Authority source stressed the data will not be used for other purposes.

'This is a one-page questionnaire which is a lot simpler than the tax return form. We will just ask about their monthly income and they do not need to report on their assets.

'The questionnaire will only be used for calculating the income index. We understand that there could be fears that the authorities could use the data to identify rich tenants and get them to pay more rent. This is definitely not the case.

'There is another system in dealing with wealthy tenants and we will not mix things up,' he said.

At present, only tenants who live in public housing flats for more than 10 years have to report their financial status regularly. Wealthy tenants are now required to pay double or market-level rents. The source said at present they estimated tenants' income by using the results of the government voluntary general household surveys. The source said a separate and compulsory study conducted by an independent research institute is needed to ensure the income index's accuracy.

The survey will be started next year before the new rent mechanism is in place.

'We need some advance data and we have to collect tenants' details anyway,' he said.

Under the proposed rent adjustment mechanism, public housing rent will be calculated based on an income index that traces the income changes of public housing tenants.

There will be an 11.6 per cent cut in rent once the new system is in place. The new rent will be the basis for future rent adjustments. There will be a review on rent every two years.

At present the Housing Ordinance caps the median rent-to-income ratio at 10 per cent.

In 2002 the authority was sued for not reducing public housing rents during deflation.

The three-year case ended last year when the Court of Final Appeal ruled the cap applied only when a decision was made to raise rents, and the cap was not a statutory definition of affordability.