During the EU referendum, fisheries minister George Eustice, who campaigned to Leave, said new powers over fisheries would "automatically" be devolved to Scotland after Brexit.
However, since the vote the UK Government has suggested it is seeking a "UK framework" for agriculture and fisheries with the support of those industries, despite the Scottish Parliament having responsibility for those areas under the Scotland Act.
At Prime Minister's Questions, Theresa May told SNP Westminster leader Angus Robertson that the government's "overriding aim" was "making sure that we don't damage the very important single market of the United Kingdom - a market which I might remind him is more important to Scotland than the European Union is."
Her comments appear to confirm that control of issues like fishing quotas and environmental standards will be passed to UK ministers rather than the Scottish Government. Mr Robertson had asked Theresa May: "With Brexit ending the role of Brussels in these areas, will all decisions about agriculture and fisheries be made at Holyrood - yes or no?"
The SNP deputy leader said farmers and fishermen had been told to expect devolution of matters affecting their industries, but "judging by the Prime Minister's answer, that that is not going to be true."
Following PMQs, Mr Robertson said: “During the EU referendum campaign, people were told that decisions currently taken in Brussels on agriculture and fisheries would revert to the Scottish Parliament - but now it seems we face a power-grab from the UK government.“The Prime Minister and the Secretary of State for Scotland failed to provide any clarity on an issue which really matters to our rural industries, our rural economy and Scotland as a trading nation."
The exchange comes after First Minister Nicola Sturgeon warned that the UK Government would try to use Brexit as an excuse to "rein in" the devolved administration during a speech in Edinburgh this week.
Earlier, during Scottish Questions in the House of Commons, Scottish Secretary David Mundell insisted Holyrood would get new powers as a result of Brexit. Mr Mundell gave an "absolute guarantee" of greater devolution, with discussions ongoing between the UK and Scottish Government on which powers will be handed to MSPs.
"This government's plan is to engage with the Scottish Government and with the other devolved administrations to discuss these very serious issues," Mr Mundell said."It's not to go out and tell the people of Scotland that the devolved settlement is being undermined by Brexit - a Brexit which will lead to more powers being exercised by the Scottish Parliament."I can give you an absolute guarantee that after the United Kingdom leaves the EU, the Scottish Parliament and Scottish Ministers will have more powers than they have today."