Clarification is urgently needed on how far the £23 million compensation scheme for the fishing sector will go following the debacle over EU exports.
Rural Affairs secretary George Eustice is coming under renewed pressure to release details on how businesses can access the scheme.
There has also been concern voiced over the £100,000 cap which has been put on claims.
Isles MP Alistair Carmichael has written to the minister.
Commenting on the letter, Mr Carmichael said: “We need immediate clarity on the detail of the compensation scheme.
“I have already heard of businesses which have taken losses saying they do not think they will be able to access the support so clear guidance is vital.
“Funds to make good the losses incurred by local businesses are welcome but they are only part of the story.”