An independent legal adviser has told councillors it would be lawful to grant permission for a 40-storey tower to rise from Brighton Marina above clifftops.

Lawyer Mary Macpherson's advice could prove decisive when Brighton and Hove city councillors meet tomorrow to decide whether the proposed 420ft tower should be approved despite a 38-year-old ban on structures higher than the cliffs behind it.

In a report commissioned by the council, Mrs Macpherson said she believed the councillors would not be acting unlawfully if they broke the height restriction in this case.

Council leader Simon Burgess, a supporter of the proposed £235 million development, welcomed the report.

He said: "What is important is that councillors can make a decision based on the individual application, this does not mean a number of tall buildings will be constructed there."

The new report was issued to councillors last week and revealed to the public yesterday.

It was commissioned in the light of criticism that the council was acting on the advice of its own legal officers and lawyers working on behalf of Brunswick Developments, the firm leading the marina plan.

The council recently granted planning permission for the scheme but will meet tomorrow to decide whether the tower will be allowed to breach the 1968 Brighton Marina Act and rise above the cliff.

In the report, which considers the formation of the act, Mrs Macpherson concludes that if the council were to refuse permission for the tower and the developers appealled against the decision, the company would "stand a high chance of success in such proceedings".

She said the firm's case would be especially strong now that the council have granted overall planning permission.

David Watkins, leader of the council's Liberal Democrat group, said he would be meeting with the chief legal officer today to discuss the report in detail.

Opponents of the development have criticised the report and suggested it had not taken all of the available documentation into consideration.

Derek Granger, chairman of the Kemp Town Society, said: "If the ruling can be broken so easily, then why was it included in the first place?"

The marina scheme is a reworking of proposals rejected by councillors last November. It will contain a swing bridge similar to the "winking eye" in Gateshead; 853 homes; a yacht club and RNLI building; a multipurpose sports court; a boules and bowling green; children's play areas; a teenage "chill out" area; a doctors' surgery; a community room and a public outdoor space.

Brunswick will give the council £1.2 million towards sustainable transport, which could help fund a rapid transport system, £875,000 for outdoor sports and £330,000 towards education.