The foreign secretary, Yvette Cooper, has accused Reform UK and the Green party of undermining Britain’s commitment to Nato.

Cooper was speaking at the Munich Security Conference, where Keir Starmer used a speech at the weekend to claim that Labour’s populist rivals, Reform and the Greens, were “soft on Russia and weak on Nato”.

“Our national security depends on us having partnerships abroad that make us strong and we have seen both Reform and the Greens undermine that commitment to the Nato alliance,” Cooper said in an interview with Sky News.

In the case of Nigel Farage’s party, Cooper said this had led to Reform “not taking seriously the threat from Russia”.

“They have refused to have an investigation into Russian interference in their own party despite the fact that their own Welsh leader was convicted of links to Russia,” she added, referring to the jailing of Nathan Gill, the former MEP and colleague of Farage, for taking bribes from a suspected Russian asset to repeat pro-Kremlin positions.

“They have too often dismissed the aggression and the threat from Russia even at the point where we have seen lethal poisons being used again as it was in Salisbury,” she said, in an apparent reference to Russia’s use of poisons that killed Dawn Sturgess in the English city in 2018 and, according to a new statement released by Britain and others, to murder Alexei Navalny, the Russian opposition leader.

The fresh attempts by Labour to attack Reform’s credentials on national security come after Farage doubled down on his past claim that the west “provoked” Russia’s invasion of Ukraine by expanding the EU and Nato eastwards.

Farage said on the Political Thinking podcast from BBC Radio 4 that he had been right and that it had been “odd to get so much abuse for being right”.

“Democracy was destroyed by those that wanted to drag Ukraine westward rather than eastward,” he said, referring to the 2014 protests that toppled Viktor Yanukovych, Ukraine’s pro-Russian president, in what Farage described as a “a street coup”.

He said last year that Ukraine should join Nato and his party supports strengthening Nato by significantly increasing UK defence spending.

Labour’s attack on the Greens came as their leader, Zack Polanski, insisted he would commit to Nato’s article 5, which commits to taking defensive military action if another member of the alliance country is invaded, but left open the possibility of leaving.

“If we’re in Nato as we are, then it’s clear that we need to sign up to the articles. And article 5 says an attack on one is an attack on all,” he told Sky News.

Polanski has advocated leaving Nato and spending less on American weapons as part of a wider dismantling of the two countries’ defence alliance, although it is not official Green party policy.