The tram project was already running well over budget
The contractors working on Edinburgh's tram line have said they want to delay the project for a further 30 months.
German firm Bilfinger Berger has also told Tie, the organisation building the line, that there will be a further increase in the cost of the scheme.
The revelation came after a meeting between project managers over the deadlock with Bilfinger Berger.
Tie said it was "deeply concerned" over the new revised programme, which it insisted was "unacceptable".
There has been speculation that tram project bosses are preparing to tear up their contract with Bilfinger Berger.
The tram project was already running behind schedule after a series of contract disputes saw workers down tools on some key stretches of the route, with costs likely to be well over the £545m which had originally been expected.
But in a statement, the Edinburgh Trams Board said Bilfinger Bergen had proposed extending the completion date by a further 30 months to 2014.
"This was deemed to be entirely unacceptable by the board, as was an unqualified increase in cost," the statement said.
The board discussed a range of options for the project, which had been due to be completed in 2012, at a meeting on Wednesday.
Board chairman David Mackay said that, while he cannot talk about the confidential details of the meeting, he was concerned for the Edinburgh people who have continued to suffer "seemingly endless delays."
He added: "I can say that today we have been presented with a series of options by the management team.
"We have had a full and constructive discussion around these options and have a clear sense of what we need to do to achieve the best possible outcome.
"We have also been reassured that the basis on which we intend to move forward is fully justified and that the enhanced rigour requested by the board earlier this year has been followed to the letter."
The board took legal and commercial advice at the meeting, and instructed the management team to "continue to apply rigorously the legal terms of the contract to the contractor including and taking into account fully their actions to date."
Mr Mackay added: "As soon as there is more to say publicly, we will provide that information.
"Our priority, and that of our partners, remains the delivery of a 21st century integrated public transport system, that represents good value and which Scotland's capital city deserves. This we will do."
Lothians MSP Shirley-Anne Somerville said: "It is absolutely essential that a way forward is found for this project that will bring an end to the problems facing Edinburgh transport as soon as possible.
"When the underground utility works are still to be completed in parts of the city it is inconceivable for anyone to now believe this project will be on time or on budget. For Leith residents it is a cruel joke to see advertising signs saying 'arriving in 2012'.
"In the next few weeks we must get a clear indication as to how much of the line will actually be completed and by when".