Russian money and Brexit are undermining the UK-US relationship. Here's how a future prime minister could improve it.

• Tension over Northern Ireland may be delaying the end of Trump-era sanctions, an ex-ambassador said.
• There is also "some resentment on both sides" over the chaotic withdrawal from Afghanistan, he said.

President Joe Biden and UK Prime Minister Boris Johnson are presenting a united front as they hold crisis talks with other world leaders over how to prevent Russia from invading Ukraine.

But the so-called special relationship hasn't always run quite so smoothly since Biden took office.

President Biden was elected months after referring to Boris Johnson as "a physical and emotional clone of Donald Trump." He has repeatedly rebuked the prime minister, both in private and public, over ongoing Brexit disputes in Northern Ireland, which have yet to be resolved, while making pointed references to his own Irish ancestry.

For his part, Johnson raised eyebrows in Washington when Downing Street said he preferred not to use the term "special relationship," suggesting it made Britain sound "needy and weak," reports said.

So what is the state of UK-US relations, and — given an ongoing row over illicit parties that could see Johnson replaced as prime minister — what priorities might help a future prime minister to improve them?

At a personal level, relations between the Johnson and Biden camps are "pretty good" at the moment, said Peter Westmacott, the UK ambassador to the US under President Obama. Defense, intelligence-sharing, and much of foreign-policy cooperation remain strong, as evidenced by the coordinated actions on Ukraine.

But there remain significant tensions, most particularly over the Brexit situation in Northern Ireland, which "may be delaying the lifting of Trump-era sanctions," Westmacott said.

Biden has repeatedly warned that Johnson's actions could undermine the Northern Ireland agreement, the defense of which he considers a key part of his political identity, given his Irish ancestry.

After the surprise resignation of Lord Frost, the Brexit minister, in December, Foreign Secretary Liz Truss was tasked with overhauling the Northern Ireland protocol. Truss, a potential leadership rival to Johnson, has already received warm praise from Brussels following a second round of bilateral talks this week, although the impasse remains.

Westmacott also mentioned "some resentment on both sides" over the chaotic withdrawal of UK and US troops from Afghanistan last year, as well as other issues, including the case of Anne Sacoolas, the wife of a US diplomat who faces criminal charges in the UK after being charged with causing the death of a teenager by dangerous driving.

The UK has also surprised some in Washington with a more muscular foreign policy than many expected it to adopt after Brexit, said Charles Kupchan, a senior fellow at the Council on Foreign Relations who was a senior director for European affairs at the National Security Council under President Obama.

"The more internationalist dimensions of the British government have prevailed against the inward-looking impulse," Kupchan said. "And I think that's good news for the UK-US relationship."

That approach has been on display in Ukraine, where the UK has adopted a harder line against Putin than many of its European neighbors as the Russian president masses thousands of troops on Ukraine's border.

The crisis appears to have sharpened minds and brought NATO powers, including the UK and US, closer together. But there is also frustration in Washington over the UK government's failure to tackle "dirty money" flowing into London from Russia.

American officials are concerned that economic sanctions in Russia in the event of war with Ukraine would be ineffective because they could launder money so easily through the UK, The Times reported.

London has gained an unwanted reputation as a money-laundering hub for prominent Russian business figures with links to the Kremlin.

"There is a level of frustration" from Washington toward the UK about the issue, Max Bergmann, a former State Department official, told Insider. "The lack of action from the UK is troubling."

Bergmann this week proposed a UK-US "anti-kleptocracy" task force should Russia invade Ukraine, a measure designed to crack down on laundered Russian money as frustration in Washington about the UK's inaction grows.

"Crises are clarifying," he said.

"If Russia does invade Ukraine, there's going to be a strong push from the US, and I think inaction from the UK would be very damaging for the relationship."

But the consensus view was that the long-term relationship between the two countries would remain strong, whatever the current leaders' personal views of each other.

Westmacott said: "President Biden showed early on that he was not going to bear grudges against Johnson. But he thinks Johnson's support for Brexit was unwise and damaging to US as well as UK interests."

"He distrusts him over Northern Ireland, and like other leaders sometimes has difficulty in knowing when to take him at his word," he continued.

"Joe Biden is a pragmatist, and he is a people person," Kupchan said. "And he believes in the importance of building relationships. I don't see any differences over Brexit or any divergences of opinion in the past as affecting the relationship."