A former UK ambassador to the US has said he expects diplomatic relations between the allies to return to “normal” now President Joe Biden is in the White House.

Lord Kim Darroch was forced to quit his stateside role in July 2019 – before Boris Johnson took the reins as Prime Minister – after frank diplomatic cables referring to Donald Trump were leaked.

Speaking about his experience under Theresa May, the life peer told MPs the relationship between Mr Johnson’s predecessor and former president Mr Trump had been “strained”.

I expect (there) to be a rather more normal and constructive relationship with the new administration

He told the Commons Defence Committee on Tuesday: “The personal relationship between Donald Trump and Theresa May was at best strained and had its acrimonious moments.”

He said a lack of notice on major US international policy shifts, such as Mr Trump’s decision to withdraw from the Paris Agreement on climate change and the Iran nuclear deal, meant the UK was often left out of the loop on matters that impacted its national security.

Mr Biden has rejoined the Paris Agreement since taking over at the Oval Office and has already held a phone conversation with the Prime Minister following his inauguration last month.

Lord Darroch told MPs that defence and security relations continued to strengthen even during the four-year Trump term.

“That aspect of the relationship continued to prosper even while things were difficult at a senior political level, and that’s a very good foundation on which to build what I expect to be a rather more normal and constructive relationship with the new administration,” he said.

He said that, with the UK hosting the next G7 summit in June and the Cop26 climate change conference in November, he expected the “channels between Washington and London to be pretty active over the next 12 months”.

The former national security adviser said he thought it would be “straightforward” for Britain to “align” with a “tougher and more challenging” position towards Russia now it had left the European Union.

His comments came on the day Russian opposition leader Alexei Navalny – one of President Vladimir Putin’s most vocal critics – was sent to prison on charges that he violated the terms of a probation while he was recuperating in Germany from nerve-agent poisoning.

But the ex-ambassador warned that further cuts to the British Army could put UK-US defence relations at risk, with Washington already concerned that the Army is too small.

Reports have surfaced that the Army could be trimmed of 10,000 soldiers as military bosses weigh up shifting to technology such as drones to replace manpower in an integrated review which is under way.

“I would be really worried about reducing further the size of the British Army,” said Lord Darroch. “If we do this then it could potentially be quite risky to our reputation in Washington.”

He said secretaries of defence in the Trump and Obama administrations used to tell him the Army was “already too small” and it had been a “big mistake” to take numbers down to 80,000.