The widow of Alexander Litvinenko, a former Russian spy turned British defector who was most probably murdered on the orders of Vladimir Putin in 2006, has said that the government has failed to learn the lesson of her husband's death.

Speaking on the Andrew Marr Show on Sunday, she said that assurances she was given by Theresa May when she was home secretary – that steps would be taken to protect British citizens from similar crimes in future – had amounted to nothing following the poisoning of former Russian spy Sergei Skripal a week ago in Salisbury.

May wrote in a letter to Litvinenko: "We will take every step to protect the UK and its people from such a crime ever being repeated."