Foreign Secretary Liz Truss is flying to Moscow to meet her Russian counterpart to urge a diplomatic end to the huge build-up of troops around Ukraine.
It is the first visit by a UK foreign secretary to the country in more than four years.
Ukraine’s ambassador to Britain earlier accused Russia of “behaving like a racketeer”, but told Sky News he believed the threat of war is lower than it was a few weeks ago.
Russia insists it has no plans to invade despite amassing more than 100,000 troops around Ukraine’s borders.
Intense diplomatic efforts have been under way to try to avoid any conflict, with Emmanuel Macron, the French leader, the latest to enter the fray when he met President Putin on Monday.
He said Mr Putin had pledged not to escalate the situation but the Kremlin later said the suggestion they would not undertake new manoeuvres near the border was “not right”.
During her two-day visit, Ms Truss will stress to Sergey Lavrov, the foreign minister, that diplomacy is the only solution and that agreements ensuring Ukraine’s independence must be respected.
Ms Truss said: “The UK is determined to stand up for freedom and democracy in Ukraine. I’m visiting Moscow to urge Russia to pursue a diplomatic solution and make clear that another Russian invasion of a sovereign state would bring massive consequences for all involved.
“Russia should be in no doubt about the strength of our response. We have said many times that any further invasion would incur severe costs, including through a coordinated package of sanctions.
“Russia has a choice here. We strongly encourage them to engage, de-escalate and choose the path of diplomacy.”
Boris Johnson is also travelling to Poland on Thursday to visit British soldiers stationed there and meet Andrzej Duda, the country’s president.
The UK said on Monday that 350 Royal Marines were being sent to the country to help local forces in the face of the tensions in Ukraine – which lies on its eastern border.
It comes after six Russian warships set off yesterday for the Black Sea – to Ukraine’s south – for planned military drills in the latest example of a concentration of forces around the former Soviet state.
‘We’re not going down without a fight’
The Ukrainian ambassador to the UK told Sky News’ Kay Burley on Wednesday that it was a “dangerous development” and that some of the vessels were assault ships.
“They’re coming, they’re massing the forces, they’re behaving like a racketeer – coming with a problem and offering to resolve it for some price… We will fight, we’re not going down without a fight,” Vadym Prystaiko added.
He said he believed conflict could still be avoided and that the situation had improved from a fortnight ago, but that his country might have to make damaging concessions to keep peace.
“I believe it’s good news that we can actually avoid war, the bad news just for Ukraine is that it might come on our expense – it will probably.”
Russia annexed Ukraine’s Crimean peninsula in 2014 and then backed a separatist insurgent in the east of the country.
Fighting between the rebels and Ukrainian forces has killed more than 14,000 people and there are fears that tens of thousands could die if Russia invades again.
Western powers have threatened unprecedented sanctions if troops cross the border and the US has said the Nord Stream 2 gas pipeline, which runs from Russia to Germany but is not yet operating, would be blocked.
The strategy behind Russia’s troop build-up is unclear – some believe it is an attempt to win guarantees that Ukraine will never be admitted to NATO, halt eastward expansion of the military alliance and force concessions from Ukraine’s president.
The Pope also commented on the crisis on Wednesday.
In remarks during his general audience, he called war “madness” and said he hoped the tensions could be reduced through “serious dialogue”.