SNP ministers are to be asked to support a new transport bill that would upset the commercial interests of the Nationalists' main financial backer.

Labour MSP Charlie Gordon plans to introduce legislation to regulate the bus network - a policy opposed by Stagecoach owner and SNP donor Brian Souter.

The policy would take power away from bus companies and give local authorities control over lucrative routes.

Gordon, who represents Glasgow Cathcart, will launch his draft Bus Regulation (Scotland) Bill when MSPs return from the recess in the autumn.

Labour believes the status quo, in which a deregulated bus network means companies can cherry-pick the best routes, leaving some areas of Scotland isolated by poor services.

Gordon's bill would give councils the power to intervene in the market and tender services through existing "quality contracts", set up by the former Labour transport minister Sarah Boyack.

The new legislation would allow local authorities to award lucrative routes to bus operators on the condition they served lower density areas as well.

The beefed-up quality contracts would also contain safeguards for workers' terms and conditions, Gordon said.

The bill is likely to put pressure on the SNP government, as the Nationalists' biggest donor runs one of Scotland's most prominent bus companies.

Brian Souter, who runs Stagecoach, provided the SNP with £500,000 to fight their successful election campaign, a donation that gave the Nationalists a financial edge over their rivals.

The Souter donation coincided with the SNP declining to include a pledge to reregulate the bus network in its manifesto.

The party had included the policy in its 2003 Holyrood manifesto, while activists also backed the move at the SNP conference three years later.

The policy was later dropped amid claims the SNP hierarchy did not want to make a promise that would alienate one of its main supporters.

Gordon said of his plans: "My bill would empower local authorities to intervene in parts of the country where there has been market failure in terms of bus provision."

He added: "I think Lothian Buses would support it, and I think First Group would listen to what I have to say. However, I think it is inconceivable that Brian Souter would agree to it, as I have been told he doesn't like similar proposals at Westminster."

A spokesman for Stagecoach said: "I would describe this proposal as unnecessary and misguided. Partnership between operators and local authorities in Scotland means bus use in Scotland is thriving. It would be tampering with a system that is benefiting passengers and which is good for the economy."

Green MSP Patrick Harvie said: "We're pleased that Labour are bringing forward a bill on bus reregulation, despite eight years of inactivity when they were in power. Just this month, parliament voted by a substantial margin for a Green amendment on this issue, so the timing is good, provided Labour's ideas go far enough and can deliver the improved bus services and reduced fares we need to see across Scotland."

An SNP spokesperson said: "We will study the details of his proposal in due course."