A CONTROVERSIAL formula that sets the toll for Hongkong's planned third harbour crossing at more than $30 and allows the operator to raise it to maintain profits will meet a barrage of criticism from legislators. A revolt against the scheme, thrashed out in Sino-British talks, approved by China last Friday and endorsed by the Executive Council yesterday, could delay completion of the tunnel until after 1997. The toll of between $30 and $40 for private cars when the Western Harbour Crossing opens is based on the calculation that the tunnel company should secure a rate of return on its investment ranging from 15 to 18.5 per cent. It is also proposed that an automatic adjustment mechanism be introduced to guarantee that the company maintains the target rate of return. If the rate of return falls below the agreed percentage, the franchise operator will be allowed to raise tolls. The toll for the two existing tunnels is $10 for private cars. The Cross Harbour Tunnel Company and the Government get $5 each from the Hunghom crossing, while the Eastern Harbour Crossing operator keeps it all. Repeated attempts to increase or vary the sharing arrangement from the current toll have been rejected by the Executive Council. Legislators warned that they would not necessarily approve the draft legislation before the current legislative session ended in July. The consortium bidding for the project said that if the franchise could not be granted before the end of August there would be substantial cost increases and slippage in the project, which is an essential part of the Chek Lap Kok airport programme. Legislators are already critical of the Government's application of the rate of return formula in deciding profit levels of other public utility companies. Anticipating opposition from Legco to extending the scheme to the new tunnel, officials have planned an intensive lobbying programme. The first step is a special briefing for legislators on Friday, when draft legislation enabling the awarding of the tunnel franchise will be gazetted. Airport Consultative Committee members will also be briefed the same day. United Democrat legislator Albert Chan Wai-yip said he did not support the notion that profits and toll levels should be determined purely by net assets. Such an arrangement had serious implications for the existing tunnels. The operators of those might ask for similar treatment, he said. Noting the consortium's concern at the urgent need to start the project, Mr Chen said even if the Legislative Council could not pass the necessary legislation before the end of July, the responsibility should not be borne by legislators. He said the Government had failed to supply relevant information earlier. It was difficult for the legislature to make a decision on such a controversial proposal within such a short period of time, he added. Independent legislator Eric Li Ka-cheung was also concerned at the formula's implications on existing tunnels. He said it would be difficult for the legislature to endorse such a proposal if it failed to take care of the public interest. Mr Li said he would not support the plan unless the Government could prove that no one would be attracted to bid for the project unless such terms were offered. ''Surely, we need a full briefing [on the plan],'' he said. Another independent, Vincent Cheng Hoi-chuen, said the plan required careful consideration. He also queried the 15 per cent rate of return, saying that if it was a minimum level guaranteed, he doubted that the Government would muster much support. It is understood that the Government maintained that granting a reasonable level of return would encourage private investment in major transport infrastructure. A product of hard bargaining between officials and the consortium, the proposed formula was accepted by the Government as it conceded that too austere a view of profitability could seriously discourage much-needed private participation in public transport projects. Should the legislation be passed, the tunnel franchise is scheduled to be awarded in August. The successful bidder for the project - a consortium comprising CITIC, Cross Harbour Tunnel Company, China Merchants and Kerry Tradings - expressed hope that Legco would give approval on time. The chairman of the Cross Harbour Tunnel Company, Gerry Higginson, said the project involved a total investment of about $7.5 billion and the consortium was looking at a certain rate of return. He warned that if the consortium did not get that rate it would withdraw its bid. ''We are looking at an investment of 30 years.' he said. CITIC Pacific managing director Henry Fan Hung-ling said: ''We have reached a consensus with the Government on the toll and its future adjustment. We're waiting for Legco to approve it. ''If we can't start by August, our contracts with contractors will expire and we'll end up having to build it at higher costs. If it's postponed to next financial year, it won't be completed by 1997.''