Britain’s military dependence on US ‘no longer tenable’, says former Nato chief

He said the UK “must rapidly pivot to becoming a more autonomous military actor”, working closely with European allies against Russia, and demonstrate progress to lifting defence spending to 3.5% of GDP by 2035 in line with a Nato target, while recognising that the US was becoming more transactional.

“The relationship with the United States will depend very much on what we contribute to the alliance,” Robertson added, amid forecasts that Germany is on track to spend twice as much on defence as the UK by 2029 if both countries maintain their current plans.

It is the second time in a week that Robertson has intervened over military spending with comments that he hopes will strengthen Ministry of Defence’s hand in a long-running row with the Treasury. There remains an £18bn funding gap in a 10-year defence investment plan, leaving key commitments not yet signed off.

The peer’s remarks came after the cross-party Lords international relations and defence committee, which he chairs, published its own report on the special relationship. It concluded that the relationship was “under greater strain today than at any point since the second world war”.

Robertson did not comment on whether the disastrous appointment of Peter Mandelson by Starmer last year had worsened a declining relationship with the US.

Lord Darroch, a former UK ambassador to Washington and another member of the Lords committee, said at the same event that it was inevitable that prime ministers would sometimes try to make a political appointment to the post.

“As a diplomat myself, I like the idea the job should, more often than not, go to diplomats. The history of British diplomats in that job in Washington has been pretty favourable,” he said.

But he added that what politicians might occasionally want was someone whom they thought they could trust, adding: “It won’t be the last time this happens.”

Responding to the Lords committee report, Warren Stephens, the US ambassador to the UK, said the White House national security strategy made it a top priority to “support our allies in preserving the freedom and security of Europe”. The US would work alongside the UK, “our closest ally”, to keep both countries and their citizens safe and prosperous, he added.