The SNP were embroiled in a "cash for policies" row last night after they dropped a key transport pledge that would havehurtthecommercial interests of their main financial backer.

The party's plans for re-regulating Scotland's buses were missing from its manifesto, which was published after the SNP accepted a £500,000 donation from Brian Souter,the owner of Stagecoach.

The U-turn has been attacked by opposition politicians who are demanding to know if the donation and policy shift are linked.

The row centres on the Nationalists' plan to reform Scotland's ailing transport network. Delegates at last October's SNP conference in Perth backed a resolution on re-regulating bus services, a policy that was also includedintheNationalists'2003 election manifesto.

However,the policy was absent when the party published its manifesto earlier this month.

The pledge to re-regulate the bus network, which would hand control of services to local authorities, has been dropped in favour of "greater integration" of the public transport network and extra investment.

ThemanifestoU-turnhasraised suspicions as it was made around the same time that Brian Souter, the owner of bus company Stagecoach, gave the SNP£500,000toboostitselection coffers.

Souter, a millionaire from Perth, is a vociferous critic of re-regulating the buses. He and his sister, Ann Gloag, capitalised on the deregulation of the network in the 1980s by purchasing dozens of firms and flooding routes with the newly purchased vehicles.

As late as last year, Souter savaged plans by a House of Commons select committee to give local authorities more control over services.

He said: "The report is nothing more than regurgitated regulation. It is a blatant attempt to renationalise the bus industry by the back door by confiscating bus companies' revenues."

The Sunday Herald also understands Souter told a Scottish minister in a meeting last year that his company would consider pulling out of the country if re-regulation was introduced.

Similarly, Souter reportedly told industry colleagues he would sooner sell his bus depots to housing developers than hand over control of services to councils.

The row is embarrassing for the SNP as it raises awkward questions about the influence of its big-business donors on party policy.

Labour was severely criticised after accepting a £1 million donation from Formula One chief Bernie Ecclestone in 1997, only then to announce the sport would be exempt from a ban on all sports sponsorship by tobacco companies.

An SNP source close to party leader Alex Salmond said the policy had been dropped over "the last few months", but said that not every policy adopted at conference automatically becomes a manifesto commitment.

Mark Ballard, the Green transport spokesman, hit out at the policy about-turn: "I'm very concerned that since BrianSouter'scashappearedinthe SNP bank account the commitment to re-regulating the bus network has disappeared from their manifesto. At best this is a downgrading of an important policy. At worst it could be cash for policies."

"The SNP must explain why a policy that received unanimous support at their last conference has disappeared."

A spokesman for Scottish Labour said: "The SNP have always pretended to be squeaky clean when it came to big donations. At the very least, questions have to be asked about whether they are in the pockets of big business, rather than doing what is best for the whole of Scotland."

An SNP spokesman said re-regulation had been dropped, but said in a statement: "The resolution stands as a statement of conference policy.The SNP manifesto expresses our costed priorities for the next four-year term.

"These priorities include £1.1 billion of investment in transport improvements,abolitionofroadtolls,and piloting a new scheme to give free bus travel."