The Foreign Office took the unusual step of naming former Ukrainian MP Yevhen Murayev as a potential Kremlin candidate to take over in Kyiv.
Western allies have stepped up warnings Russia will pay a heavy price if the estimated 100,000 troops massed on the border launch any kind of incursion into Ukraine.
Asked if he thought Russia would invade Ukraine, Mr Raab told the BBC’s Sunday Morning programme: “I think there is a very significant risk of it.
“The world needs to keep its eye on this and be very clear with President Putin that it would not do this cost-free, that there would be a price.
“A price in terms of the strenuous defence that we would expect the Ukrainians to put up, but also the economic cost through sanctions, which are of course more effective if the international community speaks as one or at least with a broad consensus.”
In a statement on Saturday, UK foreign secretary Liz Truss said the Russian plotting showed the lengths to which the Kremlin was prepared go to undermine the government in Kyiv.
“The information being released today shines a light on the extent of Russian activity designed to subvert Ukraine, and is an insight into Kremlin thinking,” she said.
“Russia must de-escalate, end its campaigns of aggression and disinformation, and pursue a path of diplomacy.
“As the UK and our partners have said repeatedly, any Russian military incursion into Ukraine would be a massive strategic mistake with severe costs.”
Vadym Prystaiko, Ukraine’s ambassador to the UK, said Ukraine was “prepared to fight”, but claimed the country was not well-equipped for a prolonged battle.
He told Sky’s Trevor Phillips On Sunday that Ukraine would “absolutely” like to join the European Union and Nato, adding: “That’s what we made clear eight years ago, and that’s why Russians came, and it didn’t change our resolve even an inch.”
Asked what he thinks the scale of the conflict could be, he said he still hopes this is just threats for now, but added the country was preparing itself.
Mr Murayev, a media owner, lost his seat in the Ukrainian parliament when his party failed to secure 5 per cent of the vote in elections in 2019.
He has reportedly spoken out in the past in support of Russia’s annexation of Crimea from Ukraine in 2014.
In addition, the Foreign Office named four other Ukrainian politicians who, it said, maintained links with the Russian intelligence services.
It said some of them had been in contact with Russian intelligence officers working on the invasion plan.
They include Mykola Azarov who served as prime minister under the pro-Moscow president Viktor Yanukovych who was toppled in a popular uprising in 2014.
Mr Azarov fled to Russia where he established what was widely seen as a puppet government-in-exile.
He has been the subject of international sanctions and an Interpol “red notice” issued at the request of the Ukrainian government.