Protestors are planning to use fish in a demonstration against the Brexit transition deal opposite Parliament today (March 21).
They are set to throw the fish into the River Thames as many have been angered that the UK is to effectively remain subject to the EU's Common Fisheries Policy in the transition period after March 2019.
Ministers have insisted that "specific safeguards" have been agreed with Brussels over the annual negotiations on fishing quotas in 2019.
However yesterday (March 20) many Tories denounced the agreement hammered out between Brexit Secretary David Davis and the EU negotiator Michel Barnier.
Craig Mackinlay and Ross Thomson, both Tory backbenchers, are thought to be taking part in the protest today (March 21).
Jacob Rees-Mogg is to also speak at a press conference, after strongly criticising the deal.
Environment Secretary Michael Gove has also talked of his "disappointment" at the EU side's unwillingness to move on the issue.
Theresa May then met with MPs who have fishing ports in their constituencies in order to explain the approach.
Meanwhile European Council president Donald Tusk has warned there is no guarantee EU leaders will accept the agreement covering the terms of Britain's withdrawal when they meet in Brussels this week.
Tusk said yesterday (March 20) that he still needs more time to discuss the matter with "some of the most concerned member states" ahead of their summit.
It had been expected that the leaders of the remaining 27 would rubber stamp the agreement, including plans for the transition period.