Edward Hodgson, managing director of Stagecoach West Scotland, said a bid to re-regulate the bus industry in Scotland would set services back 30 years.
In a letter to Green Party MSP Patrick Harvie, who founded the Better Buses campaign, he said: "We doubt whether proposed legislation to confiscate the freedom to operate commercially will comply with EU Human Rights legislation, and in any event we consider such action would be immoral."
Mr Hodgson, who runs his company's Glasgow and Ayrshire bus division, went on to say the scheme "could destroy successful businesses, would offer no protection to employee job security, pay and pensions, and would shake shareholder confidence in the Scottish bus industry".
He said the plan would burden taxpayers while leaving passengers with "higher fares or lower service levels".
October 11 is the deadline for responses to Labour MSP Iain Gray's consultation on bus regulation.
Mr Gray, a former bus conductor, wants local councils to be able to bundle together profitable and loss-making routes and offer them as a package bus firms can bid for, with fines if they subsequently drop unprofitable services.
He wants to introduce a Bus Regulation (Scotland) Bill following a series of cuts by First Group to bus services in his East Lothian constituency.
Mr Gray said: "It is no surprise to me the bus operators are not keen on local authorities having more say in services. To call it immoral is a little on the hysterical side though.
"If there is anything immoral it is the number of people who have seen their bus services disappear and the number of elderly and vulnerable people who struggle to find convenient and affordable transport in their area."
He added that while he agreed there was an "imbalance" between the amount of public money allocated to trains compared to buses, operators could not expect additional funds with no strings attached. Buses are responsible for five times as many journeys as trains, but receive only a third of the funding.
Mr Harvie said Mr Hodgson's claims were "laughable". He added: "The ludicrous suggestion that it is immoral to rein in the behaviour of the free market is followed immediately by a complaint that there is not enough public subsidy.
"This is mental gymnastics of Olympic standard. Countries that manage to provide the high quality public transport Scotland deserves do it with a mix of regulation, subsidy, and, in some cases, public ownership."
However, George Mair, the director for Scotland of the Confederation of Passenger Transport, which represents bus operators, said the proposals were "unacceptable and unworkable" and could see successful bus companies robbed of some of their most profitable routes.
A spokesman for Stagecoach said operators had already paid £90 million to buy the former state-owned bus companies.
He said: "Iain Gray's plans would be the transport equivalent of someone who had bought their council house from a local authority, spent money investing in the property over many years, having it compulsorily put up for auction and then told they would receive none of the proceeds.
"Iain Gray wants a Bill about buses, but it would mean a huge bill landing on the doormats of bus passengers and taxpayers."