A man deemed likely to help try to kill a Russian politician on Britain's streets has been allowed to return to the UK to fight to stay in the country.
Three Court of Appeal judges overturned an order blocking the 47-year-old Chechnyan from seeking a judicial review of Home Secretary Theresa May's decision to ban him from the UK and cancel his indefinite leave to remain.
The judges quashed the order despite a rare public statement from the Security Service - MI5 - which said he posed a serious threat to the life of a Russian exile living in the UK.
The decision will fuel concerns over how Britain can be protected from foreign threats and comes after a wealthy Russian banker was gunned down outside his executive flat near Canary Wharf, east London, almost two weeks ago.
German Gorbuntsov was shot a number of times as he entered his block of flats and Scotland Yard is now liaising with Russian police for the first time on a major criminal inquiry since the diplomatic fallout over the death of dissident Alexander Litvinenko.
British prosecutors named former KGB agent Andrei Lugovoy as the main suspect after Mr Litvinenko, 43, died of radioactive polonium-210 poisoning in the capital in November 2006, but the Russian authorities have repeatedly refused to send him to the UK.
In the latest case, the Security Service told the court that the man, referred to only as E1, was a former adviser to Ramzan Kadyrov, the President of Chechnya, and had "played a significant role in the assassination of (a Chechnyan exile called) Israilov in Austria on behalf of Kaydrov".
In March last year, MI5 added that Kadyrov, who had been responsible for the assassination of a number of his opponents, had a black list of individuals, some of whom he wished to have assassinated.
The exiled Prime Minister of Chechnya, Ahkmed Zakayev, a friend of actress Vanessa Redgrave who fled to London in 2002, was believed to be on the list, MI5 added.
The Court of Appeal judgment, which included MI5's assessment, added that "Kadyrov was likely to seek to target Zakayev, and that should he seek to do so in the UK 'it would be likely to be facilitated through (E1) who would be well placed to provide valuable information'.
"Therefore the appellant (E1) posed a serious threat to Zakayev's life."
But the Home Secretary's notice banning E1 from the UK and revoking his indefinite leave to remain, issued while he was outside the UK in May 2010, wrongly stated that he could not return to the UK to appeal against the decision.
In April last year, while still outside the UK, he lost a bid at the Special Immigration and Appeals Commission (Siac) for a judicial review of the decision.
But Lord Justice Sullivan, along with the Court of Appeal judges Lord Justice Moses and Lord Justice Pull, ruled on March 22 that the notice banning E1, who has a wife and six children, from the UK should be quashed.
While the incorrect notice "did not entirely deprive him of an effective right of appeal, it did deprive him of a valuable right: the right to pursue his appeal in-country", Lord Justice Sullivan said.
He added that if the Home Secretary "either failed or refused to take any steps to rectify the error, it is difficult to see how the court could properly decline to quash the defective notice".
The court heard that E1 had been granted asylum and indefinite leave to remain in the UK in July 2002.