Tory Brexiters have demanded that Theresa May reject the transition deal agreed between David Davis, the Brexit secretary, and his EU opposite number, Michel Barnier, in Brussels on Monday.

The 13 Conservative MPs and Sammy Wilson of the DUP – whose 10 MPs give May her working majority in the Commons – have signed a letter condemning the failure of the draft withdrawal agreement to end the common fisheries policy as soon as the UK formally leaves the EU on March 30 2019.

Instead, British fisheries will remain in the CFP – one of the most bitterly resented aspects of EU membership – until the end of the transition period in December 2020. In the final year of the transition agreement the UK will have no say in the policy, although the EU has said quotas will not change and there will be “consultations”.

The letter says: “The effect of ending discards during this period without compensating measures will be a further disaster for the UK’s already shattered fishing fleet, particularly for the inshore fleet, further eroding prosperity in vulnerable coastal communities. These demands are completely unacceptable and would be rejected by the House of Commons.”

The MPs are taking part in a protest on the Thames outside the Palace of Westminster on Wednesday morning. They have been joined by the former Ukip leader Nigel Farage and are throwing fish back into the river to underline their objection to the EU-imposed practice of discarding fish that do not meet quota rules.

This is the first open revolt by the Brexiters since the letter from the European Research Group signed by 62 Conservative MPs setting out their red lines a month ago. Earlier this month, Michael Gove, the environment secretary, went to Scotland to make a common declaration with the Tory leader Ruth Davidson calling for a “fast exit” from the fisheries policy.

On Tuesday, forced to answer questions from a hostile Commons on the failure to deliver on his pledge, Gove admitted he was also disappointed. But he insisted that MPs should keep their eyes on the final prize of leaving the EU altogether by 2020.