The Deputy First Minister said comments by Scottish Secretary Alistair Carmichael undermined claims about enhancing devolution.
Mr Carmichael said the UK Government would look to have a "strong Scottish voice in Whitehall and a strong UK Government presence in Scotland".
Ms Sturgeon, however, said it was contrary to claims there would be more powers. She said: "Alistair Carmichael has let the cat out of the bag about Westminster's attitude if there was a No vote and undermines the no campaign claims about more powers for Scotland.
"Rather than enhanced devolution, there would be a re-assertion of Westminster's authority. It is not possible to have both and, thanks to Mr Carmichael, the UK Government's attitude has now been laid bare."
Mr Carmichael's comments appear to show his previous call to scrap the Scotland Office and merge it into a Department for Nations and Regions was being abandoned. He said: "The nationalists had been allowed to hollow out the role of the UK Government".
Ms Sturgeon said: "Alistair Carmichael used to say that the post of Scottish Secretary at Westminster was redundant and 'indefensible", but now the UK Government's role is to be actually strengthened - and there is a real danger that the Tories could be re-elected next year, which is a risk Scotland cannot afford to take.
"That is a big reason why we need a Yes vote in September, so that Scotland will have the certainty of knowing we will get the governments we vote for every time.
"Given that the Tories govern Scotland with just a single MP here, it is the Westminster system which has hollowed out proper represen-tative democracy in Scotland."