The chairman of Edinburgh's beleaguered trams project, David Mackay, has resigned with immediate effect.
After stepping down, the transport expert, who was chairman of both Edinburgh Trams and Lothian Buses, criticised the German contractors.
In an interview with The Scotsman newspaper, Mr Mackay described Bilfinger Berger as "delinquent".
Work on the £545m system is on hold due to a dispute between the contractor and city-council owned tram developer Tie.
Mr Mackay told the newspaper: "Bilfinger Berger was a delinquent contractor who scented a victim, who probably greatly underbid and would use the contract to make life extremely difficult for the city. And they have done exactly that."
He went on: "We had found crazy things like underground chambers on Princes Street and cables were not where they should be - it was hell on wheels."
Mr McKay, aged 67, announced that he was standing down immediately without serving a notice period.
He told BBC Radio Scotland's John Beattie programme: "We are well behind the project for many sensible and understandable reasons but people ignore these and focus on the headlines.
"The trams project has been a whipping boy for a long time, but the team are very strong.
"In situations like this you meet good people, not so good people and you also meet Luddites. There have been a lot of Luddites around and my resignation brings an opportunity for a cultural change."
He said his experience with the contractors had taught him to "expect the unexpected".
"Dealing with them on a daily basis is a bit like going round Aintree with blindfolds on, on a pretty poor horse," he said.
"They're very good at camouflaging their own incredible inefficiencies."
Councillor Gordon Mackenzie, transport convener for Edinburgh Council, said he was aware of the frustrations expressed by Mr Mackay, but he was positive about the way forward given the strong management team and committed staff.
The leader of the council, Jenny Dawe, said she was "disappointed to be losing the drive, commercial expertise and commitment" which Mr Mackay had brought to Transport Edinburgh, Lothian Buses and Tie.
She added: "However, I would like to express my sincere thanks to David and pay tribute to the pivotal role he has played on this enormously important project for the city.
"I was aware that we were unlikely to retain David until full project delivery and fully appreciate his reasons for standing down at this point.
"Urgent steps will be taken to ensure that we build upon the very substantial foundation that David Mackay has built."
SNP leader and deputy council leader Steve Cardownie told BBC Scotland: "I'm deeply concerned, Mr Mackay refers to the project as 'hell on wheels'.
"That is coming from the man at the top and I think it has severe ramifications for the project.
"If the captain jumps ship and all the passengers are still on board then I think we should be worried about it.
"I want to know what Dave Mackay knows that we don't know yet."
Richard Jeffrey, chief executive of Transport Edinburgh Limited (TEL) and Edinburgh Trams, said: "It has been an absolute privilege to work alongside David during the 18 months that I have been here.
"The team is strong in all three organisations and we are resolute in meeting the demands of the task that faces us, we have excellent people in place, we are clear of the future vision for TEL."
Shirley-Anne Sommerville, SNP MSP for the Lothian region, said: "David Mackay's interview has shown this project was in chaos right from the start.
"I would like Richard Jeffreys to appear before the transport committee because I think we are entitled to know how much money has been spent and if we will ever see a tram running in Edinburgh."
Scottish Finance Secretary John Swinney has already made it clear that the tram project would not get a penny more of government funding even if it hit the financial buffers.
The dispute between Tie and Bilfinger Berger could yet end up in the courts.
But it is still hoped that the new tram network will be finished by the end of 2013.