Anthropic declined to submit its latest model, Claude Mythos 5.1, to Britain's AI Security Institute (Aisi) for testing before release, prompting fears inside the UK government of a protectionist shift among tech groups. The nearly $1tn company, gearing up for a blockbuster initial public offering, launched the model exclusively to vetted US organisations, excluding Aisi. Aisi, the global leader in testing frontier AI models, was denied pre-release access for the first time. "National security officials have concerns.

They definitely didn't test Mythos 5.1. There's anxiety around it... They are worried this could be a sign of things to come with Aisi not being given access to the latest models," said one government insider.

US President Donald Trump's administration has taken an increasingly restrictive stance on the AI industry, using sweeping export controls. The Cabinet Office is alert to both security impact and potential economic hit to Britain's economy if the country lags behind in access to cutting-edge AI models.