A row has reopened over the Scottish Executive's decision to award two shipbuilding contracts to a Polish yard earlier this month.
The SNP says UK ministers were prepared to support the yard
The SNP has obtained a letter which it claims proves the executive could have backed Ferguson of Port Glasgow.
The letter between UK Cabinet ministers expressed concern at allegations of unfair state assistance helping the Polish Remontowa yard.
Nationalists said this was a carefully worded hint to exclude the Poles.
The letter from Industry Minister Alan Michael was addressed to local MP and Scotland Office Minister David Cairns.
It confirmed the European Commission was launching an inquiry to investigate how the Polish shipyard was able to undercut the Ferguson yard.
It concluded that it was for the executive to decide how this should affect its procurement decisions.
But SNP Holyrood leader Nicola Sturgeon said it showed Westminster would have backed Scottish ministers if the £15m order went to Ferguson.
"The letter makes it clear that the Scottish Executive could have taken this investigation into the Polish yard into account when it was deciding where to send the contract," she said.
"That makes it pretty clear that even the DTI (Department of Trade and Industry) thought that any government standing up for Scottish jobs and a Scottish industry would have knocked out of the tendering process a yard under investigation and awarded the contract to Ferguson's.
"The fact that they didn't beggars belief."
Back in June, Mr Cairns said the Department of Trade and Industry was backing Ferguson's complaints against the Gdansk yard.
But Scottish Fisheries Minister Ross Finnie has insisted he had no grounds on which to exclude Remontowa.
In a statement, the executive said it had no information to suggest that there would be an inquiry into the Polish yard.
"The Scottish Executive raised Ferguson's assertions of unfair competition with UK ministers in October last year," said a spokesman.
These concerned the tender for a general lighthouse authority vessel. The UK Government at that time took the view that there was no evidence to corroborate Ferguson's complaint.
"We are not aware of any new evidence held by the UK Government or others that would change that position."
But senior Labour MSP Trish Godman said that she had been given a different message from European sources.
She said: "I am more convinced than ever that the executive could have awarded this contract for the fisheries protection vessels to Ferguson.
"I've been speaking to people in Brussels over the last few weeks and they see no reason why the two vessels couldn't have gone to Ferguson."