UK Tory leader Michael Howard has come under fire after pledging that a future Tory government would swiftly pull out of the Common Fisheries Policy (CFP).
Michael Howard addressed delegates in Dundee
Scottish Secretary Alistair Darling accused the Conservatives of "gesture politics" which he said would effectively isolate Scotland.
The Scottish National Party said talks over a European constitution should be used as a way of ending the CFP.
The Liberal Democrats said the answer was not withdrawal but radical reform.
Mr Howard is warning that he is ready to defy the European Union if necessary and pull the UK out of the CFP.
He told the party's Scottish conference that the step is the only way to save Scotland's fishing fleet.
The Tories would prefer to abandon the CFP in negotiation with other states.
Mr Howard went further at the conference in Dundee by saying he is ready to introduce unilateral legislation at Westminster in the event of resistance.
The Conservative leader argued that the CFP was emptying the seas around Scotland.
He told the conference that the policy has utterly failed and that withdrawal is urgently required.
Mr Darling told BBC Radio's Good Morning Scotland programme: "Everybody knows as far as fishing is concerned the key is to make sure we can conserve stocks and that we have an industry that is sustainable.
"This is simply gestures on the part of the Tories and the Nationalists.
"It won't help the fishermen, it won't help the fish and it would marginalise us in Europe which I believe would be disastrous for many many thousands of jobs that depend on us being in Europe and being there influencing those key decisions."
The CFP has proved controversial
Mr Darling attacked the opposition's "scepticism" over Europe, which he said would isolate Scotland.
He said: "If you abrogate the Common Fisheries Policy you abrogate the treaty the Tories signed in 1973. You cannot just unilaterally come out of it without putting our entire position in Europe in doubt."
The Scottish National Party (SNP) Westminster leader, Alex Salmond, accused Mr Howard of opportunism.
Mr Salmond said: "Michael Howard talks about fishing in Scotland, but in 126 questions to the prime minister at prime minister's questions, he hasn't mentioned fishing once."
He branded the CFP a disaster but instead called on using the negotiations on a European constitution as a way of bringing it to an end.
The Scottish Liberal Democrats said the answer was not withdrawal but radical reform.
Elspeth Attwooll MEP said: "We will fight for a decentralised management of fishing based on a regional management that will closely involve local fishermen."
She added that the fishing industry should not become a victim of "short-term populism".
Turning to other issues, the Tory leader insisted that a Conservative government at Westminster would work constructively with the Scottish Executive and the Scottish Parliament.
However, he argued that Scottish MPs should not vote on English matters in the Commons.
He also attacked the Scottish Parliament building project as "monstrous, disgusting mismanagement".
Shadow Chancellor Oliver Letwin will address delegates on Saturday.