A prominent Conservative party donor has called for ministers to oust Theresa May after failing to strike a Brexit deal.

The founder of Pimlico Plumbers, Charlie Mullins, claimed the prime minister’s incompetence is “hobbling” the UK’s negotiations and invited “that bunch of jellyfish masquerading as the cabinet” to force her from office.

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Mullins also blamed May for handing power to the Democratic Unionist party, who he describes as a bunch of “bowler hat wearing relics”.

The DUP’s intervention on Monday prevented an EU deal from going through. His intervention follows jitters among Conservative MPs over the handling of the negotiations which they say may have damaged the UK’s reputation for competence.

Quick guide Why is the Irish border a stumbling block for Brexit? Show Hide Counties and customs Inside the EU, both Ireland and Northern Ireland are part of the single market and customs union so share the same regulations and standards, allowing a soft or invisible border between the two.

Britain’s exit from the EU – taking Northern Ireland with it – risks a return to a hard or policed border. The only way to avoid this post-Brexit is for regulations on both sides to remain more or less the same in key areas including food, animal welfare, medicines and product safety. The 'backstop' in Theresa May's Withdrawal Agreement was intended to address this - stating that if no future trade agreement could be reached between the EU and the UK, then rules and regulations would stay as they are. This has been rejected by Brexit supporters as a 'trap' to keep the UK in the EU's customs union, which would prevent the UK striking its own independent trade deals. There are an estimated 72m road vehicle crossings a year between Northern Ireland and the Republic of Ireland, and about 14% of those crossings are consignments of goods, some of which may cross the border several times before they reach a consumer. Brexit supporters say this can be managed by doing checks on goods away from the border, but critics say it will be difficult to police this without any physical infrastructure like border posts or cameras, which could raise tensions in the divided communities of Ireland.

Interactive: A typical hour in the life of the Irish border Photograph: Design Pics Inc/Design Pics RF

May had to break off from lunch with the European commission president, Jean-Claude Juncker, which was due to settle the border issue to take an urgent call from Arlene Foster, the DUP leader, whose party supports her minority government in Westminster.

In a post published on his blog, Mullins wrote: “Yesterday proved beyond doubt that Theresa May is not only weak but that it’s her incompetence that is hobbling the UK.

“It was her ludicrous power grab in calling a snap general election that not only destroyed the healthy Tory majority, but, as we saw yesterday, handed extreme power to a bunch of bowler hat wearing relics who’re still fighting a 300-year-old war.

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“There is no way the CEO of any company could show such disastrous judgment, and not be ousted by the board, and that is why I’m calling for that bunch of jellyfish masquerading as the cabinet of the United Kingdom need to get some balls and get her out.

“This is no longer about leaving or remaining, this is about having strong and credible government, and Theresa May has neither the power to do a good Brexit deal for the UK nor the authority to call off the madness. She must go for all our sakes.”

Mullins, whose south London-based firm has given £16,000 to the Tories this year and donated more than £70,000 while David Cameron was leader, has been a critic of May’s policies on the EU.

Last year, he was one of a number of donors who funded the successful legal challenge question to the government’s legal advice that article 50 can be invoked under the royal prerogative, which does not require parliamentary approval.