The firm behind plans for a Forth hovercraft service has accused politicians of using the project as a "political football".
Stagecoach has already trialled the hovercraft on the Forth
Stagecoach said it was putting the plan on hold until it got "clarification" on the public sector's investment.
A spokesman accused unnamed politicians of seeking to make "political capital" out of donations from the firm's co-founder Brian Souter to the SNP.
Lothians Labour MSP George Foulkes branded the remarks "astonishing".
Mr Souter, who set up the transport group in Perth in the 1980s, gave £500,000 to the SNP before the elections last year.
A spokesman for Stagecoach said it was putting research and development on hold from the end of this month.
He said: "We have been entirely transparent in discussing the business case for the hovercraft project with many stakeholders and we would expect the same rigorous examination and financial assessment of our proposals as any other transport project.
"However, we have become increasingly frustrated and angry at the cynical use of the hovercraft project as a political football.
"Some politicians have put personal and party self-interest before the communities in Fife and Edinburgh.
"The fact is that we have received support for the hovercraft concept from both the current Scottish Government and the previous administration, which approved public funding to meet part of the cost of the two-week trial in July 2007."
A two-week trial which took place last year between Kirkcaldy and Portobello in Edinburgh was said to have established a business case for a full-time route.
After analysing the results of the £300,000 pilot scheme, the firm said its preferred permanent route was between Kirkcaldy and Leith, slightly to the west of Portobello.
Last October the firm said a full service could be operational from summer 2009, with a one-hovercraft service operating between Kirkcaldy and Portobello as an interim measure from late this year.
The service would involve a £10m investment by the Stagecoach group with short-term public sector support of about £3.3m for the first three years.
Mr Foulkes said: "When you're dealing with over £3m of taxpayers' money as a public subsidy to a company owned by one of the SNP's largest donors, the highest possible standards of scrutiny and accountability must apply, and European rules on tendering adhered to.
"I had a very constructive meeting with Stagecoach just two weeks ago where I expressed my support for the project in principle, and made some suggested amendments to their plan which they said they considered helpful.
"The remarks made by Stagecoach today are therefore astonishing and I can only assume that once they realised they could not be given a privileged position because of their owner's links to the SNP, they decided to find an excuse to justify their decision to backtrack."
Russell Imrie, South East Scotland Transport Partnership (Sestran) chairman, said: "When Sestran met Stagecoach, we expressed our support for the project, but both parties recognised that much more work needed to be done on the business plan, before any commitment for public funding could be made.
"Stagecoach agreed to develop the plan further and we are surprised at this sudden turn of events."
A Scottish Government spokesman said: "The Scottish Government is keen to see progress on this project, which needs to be considered in the context of the wider study on cross Forth travel options being undertaken by Sestran, Fife and Edinburgh Councils.
"Once we receive it, we look forward to studying a final business case from Stagecoach."