A video of defence secretary Ben Wallace being duped into speaking by phone to an impostor posing as the Ukrainian prime minister was published on Monday – hours after Downing Street said it believed Russian state actors were responsible for the hoax. In the short clip, Wallace replies with scepticism and apparent confusion when the caller asks him questions. Earlier, the UK government warned that the impostors were linked to the Kremlin and could release a doctored video of the call with the defence secretary. Wallace tweeted: “Things must be going so badly for the Kremlin that they are now resorting to pranks and video fakes. Not the actions of a confident Government, but then again after the Salisbury Cathedral sightseeing story, anything is possible …” The prankster pair, Vovan and Lexus, claimed responsibility for the video. They are suspected of links to Russia’s security services, which they have denied. The episode raises serious questions about the security and defence failings that led to pranksters getting through to one of the UK’s most senior ministers and managing to get him to talk about a highly sensitive military and diplomatic situation in Ukraine. In the clip, Wallace appears to be asked if he will support Ukraine’s nuclear aims – Russia has falsely claimed Kyiv has nuclear ambitions. A defence source said: “It’s a doctored clip. What you don’t hear is the defence secretary also saying that the UK can’t have anything to do with alleged Ukrainian nuclear ambitions, because the UK is committed to the nuclear non-proliferation treaty.” Government sources said the video was doctored but did not dispute the substance of what Wallace appeared to have said. Vovan and Lexus are professional prank callers, whose real names are Alexei Stolyarov and Vladimir Kuznetsov. They have previously got through to Elton John and Turkish president Recep Tayyip Erdoğan. The caller who reached Wallace was pretending to be Ukraine’s prime minister, Denys Shmyhal. Priti Patel, the home secretary, was also targeted, as well as Nadine Dorries, the culture secretary. In its first statement attributing blame for the video calls, No 10 said on Monday it believed Russian state actors were responsible. The prime minister’s official spokesperson said: “The Russian state was responsible for the hoax telephone calls made to UK ministers last week. “This is standard practice for Russian information operations and disinformation is a tactic straight from the Kremlin playbook to try to distract from their illegal activities in Ukraine and the human rights abuses being committed there. “We are seeing a string of distraction stories and outright lies from the Kremlin, reflecting [Vladimir] Putin’s desperation as he seeks to hide the scale of the conflict and Russia’s failings on the battlefield.” Wallace publicly acknowledged he had been targeted shortly after his call on Thursday. An investigation has been launched into how it happened and a cross-Whitehall review is looking at tightening up security procedures. There have been many similar hoax calls in the past but not attributed to the Russian state. In May 2018, Boris Johnson – then the foreign secretary – talked about international relations and rude poetry with a hoax caller who pretended to be the Armenian prime minister. In 2015, it emerged that an impostor claiming to be the head of GCHQ, Robert Hannigan, managed to get through to the then prime minister, David Cameron, on his mobile number. In another hoax, a caller rang GCHQ and managed to obtain Hannigan’s mobile phone number.