London Mayor Sir Sadiq Khan is inviting San Francisco-based artificial intelligence (AI) developer Anthropic to grow its footprint in his city, while criticizing the Trump administration for seeking retribution against the company.

Sir Sadiq's letter to Anthropic boss Dario Amodei came as the Pentagon designated Anthropic a supply chain risk after Amodei refused to give US defence agencies unfettered access to its AI tools.

"It is extremely concerning to see this kind of behaviour, which is a clear attempt to intimidate and punish Anthropic for refusing to remove ethical safeguards," Sir Sadiq wrote.

Anthropic said it planned to challenge the designation in court.

Asked about the mayor's letter and his offer, a White House spokesperson told the BBC: "As President Trump said, we will never allow a radical left, woke company to dictate how our United States Military fights wars".

The BBC has contacted Anthropic. A spokesperson for the Mayor's office told the BBC that there have been discussions this week with "senior leaders" of the company.

In talks with US Secretary of Defence Pete Hegseth, Amodei took issue with the prospect of having Anthropic's Claude model used to conduct mass domestic surveillance or autonomous military targeting.

The Pentagon has said the military must be able to use technology "for all lawful purposes" and argued those feared uses would violate the law and were not planned.

Last week, US President Donald Trump said he would direct every federal agency to immediately stop using technology from AI developer Anthropic.

"I applaud your steadfastness in the face of such pressure," Sir Khan wrote. "[I]t would be good to discuss how we could support you to expand operations further."

He added that London "could provide an even more significant location and platform for the future of Anthropic".

Sir Sadiq's letter could position London as a viable alternative for Anthropic as the company's relationship with the US government continues to deteriorate.

The AI developer had been in negotiations with the Department of Defence, but on Thursday, Under Secretary of Defence Emil Michael said talks were off.

"I want to end all speculation: there is no active [Department of War] negotiation with [Anthropic]," Michael wrote on X.

The Department of War is a secondary name given by Trump to the Department of Defence.

The Pentagon's designation is the first time a US company has been labelled a supply chain risk, meaning the government considers Anthropic not secure enough for its use.

When the Pentagon first threatened the designation, insiders at Anthropic feared it might affect the company's business with partners that also conduct business with the US government.

So far, those fears appear unfounded.

On Thursday, tech giant Microsoft said it would continue to embed Anthropic technology into its products for clients, with an exception for the US Department of Defence.

"Our lawyers have studied the designation and have concluded that Anthropic products, including Claude, can remain available to our customers," Microsoft told the BBC in a statement Thursday.