Two of Theresa May’s most senior ministers are feuding over the U.K.’s fishing rights after it leaves the European Union, as the prime minister struggles to keep control of a Cabinet divided over Brexit.

David Davis, Britain’s chief Brexit negotiator, is furious with Environment Secretary Michael Gove for publicly criticizing the transition deal struck with the EU last month, according to two people familiar with the situation.

The row matters because Gove and Davis are two of the most influential pro-Brexit ministers in May’s inner circle. As the clock ticks down to the divorce in March next year, tensions between key Cabinet colleagues threaten to make it harder for the premier to reach a political agreement within her team so she can make the compromises necessary to achieve a deal with the EU.

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The main argument now is between pro-European ministers who want the U.K. to stay in the EU’s customs union, and Brexiters including Davis and Gove who want to leave it. The emergence of splits even among the pro-Brexit faction—over an apparently secondary issue such as fishing—take the Cabinet division to a new level.

On March 19, Davis and the EU’s chief negotiator, Michel Barnier, announced that they had agreed to a 21-month transition phase to help smooth the Brexit journey for citizens and businesses. The deal was seen as a crucial success for May and marked a key milestone on the road to Brexit.

‘Disappointment’

But when called to Parliament the following day to explain why the EU would continue to have control over British waters until 2021, and the government in London would have less of a say than it does as a member of the bloc, Gove said he shared the “disappointment” of the British fishing industry.

“We didn’t get everything we wanted,” Gove told the House of Commons.

Davis is said to have told allies of his anger at Gove’s remarks, because the environment secretary had long known that it would be impossible to persuade the EU to grant Britain full control of its waters until after the transition period. Allies of Davis accuse Gove of trying to deflect the blame onto him.

Many Scottish members of Parliament—for whom fishing is a key industry—take a different view, and have told the government that they share Gove’s apparent frustration with Davis and his team. One Conservative Party member says the U.K. should have been more ambitious in its talks with the EU.