Reform UK’s candidate in the Makerfield by-election said Russia was “within their rights” to invade Crimea in 2014.
In an online forum, Robert Kenyon agreed with a post that described the annexation of Crimea as “democracy in action”.
Under a post titled “Hypocrisy of the West regarding Ukraine in the sin bin”, one forum member wrote: “The people of the Crimea want to be in Russia, for me that is democracy in action.
“The government should work for the people, not the other way round. The people have spoken and they have what they want. The Falklands and Gibraltar, they want to stay British, so be it.”
Mr Kenyon replied: “I agree totally, Russia are well within their rights to do what they have done, as we did with the Falklands. However, will Latvia be next?”
It is the latest in a string of historical comments made by Mr Kenyon that could damage Reform’s campaign against Labour’s Andy Burnham in Makerfield. Last week, a poll showed Reform trailing Labour by only 3 per cent in the constituency.
On Tuesday, Reform issued a statement insisting that Mr Kenyon was a “proud Brexiteer” and had never endorsed the EU’s freedom of movement policy.
The intervention followed the emergence of historic social media posts in which the plumber, who is challenging Andy Burnham for the Leave-voting seat, appeared to express doubts about the decision to leave the EU.
In one message, he wrote: “So anyone who thinks I love Trump, voted Brexit, read the Daily Mail, live in the 1950s, a Tory and 103 is wrong. I’m none of the above.”
In another discussion titled “Brexit Anyone?,” Mr Kenyon wrote: “I woke up the day after Brexit s—ing myself to what was voted for.”
Last week it emerged that Mr Kenyon had reshared comments by far-Right influencers with a history of Holocaust denial and was Facebook friends with a neo-fascist.
He also posted offensive comments about women, which included expressing his support for a sexually explicit tweet about Carol Vorderman.
Labour’s Andy Burnham leads Reform by 3 per cent in Makerfield, latest polling shows – Ian Hodgson/AP
Mr Kenyon’s remarks about Russia were made a month after the annexation of Crimea in February 2014.
A referendum was then held the following March, in which Russian election officials claimed 95.5 per cent of voters supported joining the Russian federation. William Hague, the then foreign secretary, described the vote as a “sham and perverse democratic process”.
In the following two years, more than 100,000 Ukrainians fled Crimea as the Kremlin tightened its grip on the region by escalating its campaign against dissidents.
Luke Pollard, a defence minister, said: “Nigel Farage has again chosen a candidate who promotes Kremlin talking points and makes excuses for Putin’s unacceptable actions against Ukraine.
“While we stand with Ukraine in the face of Russian aggression, Robert Kenyon has shown he’s completely out of step with the British people.”
A Reform spokesman said: “At no point did Rob explicitly support or endorse Russia’s actions in Crimea. He is fully opposed to Russia’s illegal and brutal invasion of Ukraine. We fully back Cllr Kenyon. He is an excellent local candidate, who we are confident will be a superb MP for Makerfield.”