PUBLIC estate tenants have threatened legal action against the Housing Authority for charging its wealthier residents twice the normal rent. They claim the policy is a breach of tenancy contract and are demanding a change in existing laws so that the authority can be monitored by the Legislative Council. A Legco motion calling for the double-rent policy to be scrapped was passed last month, but the authority argues that the decision had no legally-binding power. Under the policy, tenants who have lived in public flats for more than 10 years are required to pay twice the normal rent if their monthly income is more than double the limit for public housing. More than 100 demonstrators yesterday petitioned the authority before members met at its Ho Man Tin headquarters to discuss the policy. A spokesman for the Anti-Double Rent Joint Concern Group, Mr Leung Ping-wah said: ''There was no such a term as 'double-rent' in our tenancy contracts and the authority has no right to add the condition to the contracts without our consent. ''We may resort to legal action to fight the policy.'' But an authority spokesman said there was a clause in the contracts that empowered the authority to adjust rents by giving tenants one month's notice. The authority received about 100 written submissions from concern groups and tenants' groups during a three-month consultation, which ended last month. Most were against the policy. Meanwhile, the Director of Housing, Mr Fung Tung, has warned that people wanting public housing units might have to wait longer if the double-rent policy was abolished. His comments were immediately criticised by legislators of the Housing Panel as an excuse invented by the Government to continue with the policy. At the moment, the number of public housing flats is about the same as the number of Home Ownership Scheme flats. However, Mr Fung said: ''If the authority experienced financial difficulty we might raise the percentage of flats built for sale to ease the burden. ''This may cause applicants for public housing to wait longer.'' In a related development, the researcher responsible for a recent independent survey on the policy, Hongkong Polytechnic principal lecturer Mr Lui Ping-keung, admitted that they had not thought of asking how much the key tenant received from their children's salaries. Legislators challenged whether the Government had deliberately avoided raising the question in the survey, which would be a key reference for the Housing Authority to decide if the double-rent policy should be scrapped. But Mr Lui argued that even if he had thought about the question, he would not have included it in the survey.