Senior politicians and animal welfare groups have condemned the government over a deal that allows meat from lambs slaughtered without being stunned to be exported to Saudi Arabia.

They say the deal, estimated by the government to be worth £25m over the next five years, shows a shameful disregard for animal welfare and signals that after Brexit it will become acceptable to lower standards to secure trade deals.

When the food minister George Eustice announced the deal in February, it was unclear whether the animals would be stunned before being slaughtered in the UK. But the only UK scheme approved by the Riyadh-based Gulf Accreditation Centre to certify the exported meat is the Halal Monitoring Committee (HMC), which forbids all forms of stunning, Vet Record magazine reported.

The Liberal Democrats’ food spokesperson, Tim Farron, said: “The government has shown extreme negligence by announcing such a significant deal without full consideration of the details. This does not bode well, as post-Brexit the UK will inevitably receive pressure from new trading partners to open markets to low-welfare animal products.”

Under EU and UK law, all animals should be stunned – usually with a bolt gun to the brain or electrocution ­– before being slaughtered, so they are unable to feel pain. But there is an exemption that allows non-stun slaughter for meat intended for religious communities within the EU. MPs and animal welfare campaigners believe the deal to supply meat to the Saudis at least goes against the spirit of this exemption.

The Labour MP Kerry McCarthy, a former shadow environment secretary, said: “The exemption is clearly there to meet the needs of our own religious communities, not those abroad.

“This deal shows that we’re already prepared to lower our standards where trade deals are concerned. It gives me little confidence that [after Brexit] we won’t find our higher standards on food safety, animal welfare and the environment bargained away when it comes to negotiating trade deals with the likes of the US.”

Ian Woodhurst, World Animal Protection’s UK farming campaigns manager, said the deal was unacceptable. “We haven’t even left the EU yet and already Defra [the Department for Environment, Food and Rural Affairs] appear to be condoning deals that threaten the UK’s reputation for high animal welfare standards. This sends a signal that post-Brexit it will become acceptable to lower animal welfare standards to secure trade deals.”

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Marc Cooper, the head of the RSPCA’s farm animal department, said 84% of halal meat in the UK was stunned before slaughter. “This [deal] is very disappointing news. Non-stun slaughter causes serious suffering for animals.”

The Green party’s animal welfare spokesman, the MEP Keith Taylor, condemned the government’s decision to “misapply EU rules to facilitate a multi-million-pound deal”.

He said: “This is a clear sign that Conservatives see no issue in sacrificing animal welfare standards on the altar of free trade once they yank Britain out of the single market.”

A Defra spokesman said: “All lamb exported to Saudi Arabia must be accompanied by a halal certificate, and all slaughter, whether stun or non-stun, must strictly comply with EU and UK rules on animal welfare. The deal with Saudi Arabia does not signal a move to non-stunned slaughter exports as the UK leaves the European Union.”