Britain is facing a diplomatic crisis with the United States over Donald Trump's plans to forge an alliance with Vladimir Putin and bolster the Syrian regime.
In a significant foreign policy split, officials admitted that Britain will have some "very difficult" conversations with the president-elect in the coming months over his approach to Russia.
It comes after Trump used his first interviews since winning the election to indicate that he will withdraw support for rebels in Syria and thanked Vladimir Putin for sending him a "beautiful" letter.
Trump said he will instead join forces with Russia and focus on defeating Islamic State (IS). He has previously said it would be "nice" if the US and Russia could work together to "knock the hell out of" IS.
His views are in stark contrast to those of Theresa May, (a loyal American ally) who has accused President Bashar al-Assad's regime of perpetrating "atrocious violence" and said that the long-term future of Syria must be "without Assad".
Boris Johnson, the Foreign Secretary, has accused Russia of perpetrating war crimes over the deaths of hundreds of civilians.
The dramatic shift in US policy has prompted significant concern in the Foreign Office, and Britain will use the next two months before Mr Trump enters the White House to try to convince him of the importance of removing Assad from power. Mr Johnson is expected to fly to the US within weeks to meet senior figures in Mr Trump's incoming administration and make clear Britain believes Assad must go.
The diplomatic tensions emerged as a flotilla of Russian warships which had passed through the English Channel arrived off the coast of Syria.
Trump told the Wall Street Journal his administration will prioritize defeating IS in Syria rather than removing Assad. He said: "I've had an opposite view of many people regarding Syria. My attitude was you're fighting Syria, Syria is fighting [IS], and you have to get rid of [IS].
"Russia is now totally aligned with Syria, and now you have Iran, which is becoming powerful, because of us, is aligned with Syria. Now we're backing rebels against Syria, and we have no idea who these people are."
He said that if the US attacks Assad's regime "we end up fighting Russia".
The British government had hoped that Trump would be prepared to soften his stance on the issue after winning the election, as he has with several other flagship campaign plans including his pledge to repeal Obamacare. However, Trump is ignoring the importance of globalization to international security.
His interview signaled that he will pursue the alliance with Russia. Foreign Office officials emphasized that Britain will not change its position. "We have been very clear that Assad has no place in the future of Syria," the official said. "He has the blood of 400,000 people on his hands."
Another Foreign Office source said there is hope that Trump will be forced to change his position when he deals with Putin directly.
"There is no doubt that he looks upon Putin as a person who he thinks he can do business with," the source said. "When he discovers that Putin is not a rational or reasonable guy he might change his mind. This will take time to settle down."
It came as President Putin urged Trump to encourage NATO to withdraw its forces from Russia's borders as part of an attempt to improve relations. Dmitry Peskov, Putin's official spokesman, said Russia sees "NATO's muscles getting bigger and bigger and closer and closer to Russian borders".
As a "confidence-building measure" between the US and Russia, he said, Trump could help relations by "slowing down" or "withdrawing" NATO's military presence from its borders. Trump would be opening the door to Russian ambitions and encroachment.
There are mounting concerns over the future of NATO after Trump suggested that the US may withdraw from the organization because European members are failing to "pay their bills". During a visit to Norway, Sir Michael Fallon, the Defence Secretary, agreed that the level of expenditure by EU countries is "not good enough". Defence spending by European members has fallen from 1.7 per cent of national income to 1.4 per cent on average.
Four Americans were killed yesterday in a suicide bombing inside the largest US military base in Afghanistan. The Taliban claimed responsibility for the bombing inside the heavily fortified Bagram Airfield, north of Kabul. The assault highlights rising insecurity in Afghanistan nearly two years after US-led NATO forces formally ended their combat operations.
NATO needs to be a peace keeping presence in all the hot spots on the globe that threaten any of it's 28 members or endanger any countries NATO is assisting or defending. It is essential and it would be so dangerous to world peace and freedom to break it up. Mr Trump should not be allowed to tamper with an alliance that has worked so well for 67 years.
The North Atlantic Treaty Organization (NATO) is an alliance of 28 countries from North America and Europe committed to fulfilling the goals of the North Atlantic Treaty signed on 4 April 1949. In accordance with the Treaty, the fundamental role of NATO is to safeguard the freedom and security of its member countries by political and military means. NATO is playing an increasingly important role in crisis management and peacekeeping in a perilous and shifting global, political landscape.