The Prime Minister made the promise in the Commons on a day when MPs joined a protest over the UK remaining in the “disastrous” common fisheries policy during the Brexit transition.
A fishing boat with Nigel Farage aboard sailed up the Thames as part of a protest to be met by pro-Brexit and coastal MPs led by Jacob Rees-Mogg.
The former Ukip leader helped to dump a tray of haddock into the Thames as a symbol of the tons of fish that are thrown overboard each year to allow for fishermen from other EU countries to plunder British stocks.
Mr Rees-Mogg, who did not board the vessel, said he believed Mrs May had made a mistake on the deal, which will lead to Britain effectively remaining in the EU’s common fisheries policy until the end of 2020.
On what Mrs May could tell the Commons to reassure them about the agreement, he said: “I think it’s very important that the rules between our formal date of leaving and the end of the implementation period do not have an unfair effect on our fishing communities.
“I would prefer it, of course, if we left properly on March 30, 2019.”
Mr Rees-Mogg said the protest was an effort to highlight the “plight” of fishing communities.
He added: “We need to ensure that we have a vibrant fishing community and we want to make sure that it survives to the end of the implementation period to benefit from our being free of the tyranny of the European Union.”
Later, at Prime Minister’s Questions, Mrs May was challenged by Tory backbencher Mark Francois, who said Britain should not rejoin the “disastrous” common fisheries policy.
Mr Francois called for reassurance that the UK would “absolutely and unequivocally” be taking back control of its waters in 2021.
Mrs May replied that the implementation period after Brexit day in March 2019 ensures businesses, including fishermen, “don’t face two cliff-edge changes” in the way that they operate.
She added: “One of the elements we’ll be looking for is to ensure we do take back control of our waters.”