FARES on Kowloon Motor Bus vehicles seem set to rise by well above the inflation rate this year, a move which could spark a fierce political debate. Legislators voted narrowly last month to recommend the fare rise be kept below inflation, 8.5 per cent last year. KMB has asked for a 19.6 per cent rise. But yesterday the Transport Branch recommended KMB fares should go up 12.9 per cent on average. The plan was endorsed by the Transport Advisory Committee last night, and it now goes before the Executive Council. The figure was criticised by Legco's two largest political parties. Legco transport panel chairman and Liberal Party member Miriam Lau Kin-yee said: ''12.9 per cent is still too high. I would be disappointed if Exco agreed to this. ''According to my calculations, 10 per cent would be more reasonable. We will make our views very clear.'' United Democrats transport panel member Lau Chin-shek said: ''10 per cent would still be too high. The increase should be below the rate of inflation.'' Advisory committee chairman Professor Leung Chi-keung said the increase was ''fair and reasonable''. ''We have taken public reaction into account. The public will see we have not approved a 19.6 per cent rise.'' A 12.9 per cent increase would give KMB a rate of return on its assets of 11.5 per cent, said Professor Leung, 4.5 per cent below the permitted maximum under the company's scheme of control. ''This is the lowest return in recent years,'' he said. KMB remained tight-lipped last night. ''It is still too early to comment. We will wait until Exco makes its decision,'' public relations manager Winnie Ng Wing-mui said.