Alex Salmond has described the decision to send a Conservative peer to represent Scotland's fishermen at European Union (EU) talks as "absurd".
The Scottish Government wanted its Environment Secretary Richard Lochhead to stand in for the UK Environment Secretary Liz Truss, who is unable to attend the meeting in Brussels.
But the UK Government has instead drafted in Lord de Mauley.
The First Minister has written again to David Cameron following confirmation that the peer will represent the UK at the Agriculture and Fisheries (Agrifish) Council meeting today.
It follows a letter to the Prime Minister at the weekend urging him to reconsider the assignment.
The talks will focus on quotas for deep sea stocks, which affect fisherman in Scotland where the majority of the UK's fishing industry takes place.
The Scottish Government said that in 2010 it was agreed that it is "reasonable, given Scotland's overwhelming interests in fisheries, that Scottish ministers could speak for the UK delegation".
Mr Lochhead told BBC Radio Scotland that he had to brief Lord de Mauley on the issues but would not be allowed to actually take part in the meeting.
"I think that is a bit of a snub for Scotland and a slap in the face," he said.
In his letter to Mr Cameron, Mr Salmond wrote: "It is simply absurd that you are placing the interests of this vital Scottish industry in the hands of an unelected peer, allowing him to make representations on areas in which he has absolutely no experience or prior involvement, rather than the Cabinet Secretary who has over seven years of experience.
"Not only does this break your personal undertakings given in 2010, it makes a total mockery of your more recent pledges to Scotland during the referendum campaign.
"Richard Lochhead will be in Brussels for the council meeting and remains ready to take on leadership of the delegation in the fisheries talks and only for the fisheries talks.
"Your Government has already demonstrated why it cannot be relied upon in EU negotiations. I again ask you to redeem your previous undertakings on an area of key importance to Scotland."
Lord de Mauley is parliamentary under secretary of state for natural environment and science and is responsible for the business of the Department for Environment, Food and Rural Affairs (Defra) in the House of Lords.
Mr Lochhead and UK fisheries minister George Eustice attended the last meeting of the Agriculture and Fisheries Council in October, where a rule change was agreed allowing fishermen affected by the Russian food export ban to bank a quarter of their mackerel quota for use in 2015.
A Downing Street spokeswoman said: "The Government takes its responsibility to act for all parts of the UK extremely seriously. Our ministers put our country's interests first and foremost in all they do.
"We have consistently and successfully worked with the Scottish Government to argue for and to secure the best deals for our fishing industry. Their hand is strengthened by being part of the UK."
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