LONDON — The cross-party Lords international relations and defence committee published a report on Wednesday warning that the United Kingdom's special relationship with the United States is under greater strain today than at any point since the second world war. The committee, chaired by former NATO secretary general George Robertson, urged Britain to rapidly move toward greater military autonomy.

"Britain's high military dependence on the US is no longer tenable," Robertson said. He said traditional allies were diverging over values and that even after Donald Trump leaves the White House, the separation between the U.K. and the U.S. was likely to continue.

Robertson said Trump carried out an unprovoked attack on Iran, decided to levy tariffs on traditional allies, and threatened to wrest Greenland from Denmark. "All of these illustrate a growing divergence between Westminster and Washington," he said. He said the diplomatic tone from the White House had reached a historic low point and that Trump's unilateral actions indicated the postwar era in which the U.S. acted as a steward of global rules, norms and institutions may well be over.

He said Trump did not represent the full spectrum of American opinion, but that the president's behaviour was reflective of longer-term changes in U.S. foreign policy. He said it was a naive belief that the White House would always be on hand to help the U.K. in times of conflict, and that such an approach had led to a diminishment of British military capability. He also said the U.K. must recognise that the U.S. was becoming more transactional.

"The relationship with the United States will depend very much on what we contribute to the alliance," he said. He said the U.K. must work closely with European allies against Russia and demonstrate progress in lifting defence spending to 3.5% of GDP by 2035, in line with a NATO target.

It was the second time in a week that Robertson had intervened over military spending. He said he hoped his comments would strengthen the Ministry of Defence's position in its long-running row with the Treasury, where an £18bn funding gap remains in a 10-year defence investment plan. Germany is on track to spend twice as much on defence as the U.K. by 2029 if both countries maintain their current plans.

Prime Minister Keir Starmer has offered limited military support to the U.S. bombing of Iran while refusing to participate directly. Trump complained that the U.K. wanted to help secure the strait of Hormuz only once the war was over.

Warren Stephens, the U.S. ambassador to the U.K., said the White House national security strategy made it a top priority to "support our allies in preserving the freedom and security of Europe." Stephens said the U.S. would work alongside the U.K., "our closest ally," to keep both countries and their citizens safe and prosperous.