A British Government minister has urged the defence industry to intervene in the Scottish independence debate, prompting SNP concern about "underhand" activity.
Philip Dunne, minister for defence equipment, told the Financial Times: "I would like to see the defence industry in Scotland being a bit more upfront in explaining their concerns to their workforce and the people in Scotland and I urge them to do that at every opportunity."
It was also reported that a senior defence executive said UK officials were making "deft use of the dark arts".
Both sides of the referendum debate frequently urge wider involvement, but the SNP claims Mr Dunne is trying to put pressure on companies to speak on behalf of the "No" campaign.
Angus Robertson, the SNP's leader at Westminster, said: "These damning revelations expose the fact that UK Government ministers are attempting to pressure companies which depend on MoD orders and contracts to speak out on behalf of the 'No' campaign.
"We need to know which ministers and officials are involved, and what exactly they are doing and saying to exert this pressure."
The SNP is tabling a series of questions at Westminster and using freedom of information rules to try to get "full disclosure" about any wider activity in other sectors of the economy.
"We now have clear evidence of a furtive whispering campaign both at home and abroad in support of the 'No' campaign, utilising public money, by a UK Government whose Prime Minister is too frightened to debate First Minister Alex Salmond face to face. It is clear that the 'No' campaign is badly rattled," Mr Robertson said.
A Better Together spokesman said: "The SNP really do take the biscuit. Accusing people of pressurising business when they have bullied and intimidated their way across Scotland for years would be hilarious if it weren't so serious."