Alexander Litvinenko told his father "Daddy, Putin has poisoned me" from his deathbed, the inquiry into the spy's killing has heard.
In a statement to police in 2006, Walter Litvinenko told how his son was struggling to speak when he visited him in hospital but accused Russian president Vladimir Putin over his death – a claim the Kremlin has always denied.
Mr Litvinenko was said to have told his father: “Daddy, Putin has poisoned me.”
Mr Litvinenko snr said: “He said Putin was a man with a perverted mind, a very dangerous man.” He said his son told him: “Daddy, be careful.”
However, Mr Litvinenko snr later spoke of his regret at blaming Mr Putin and the Russian government for the killing and retracted the allegations he had made against them.
In an interview with Russian television in 2012, he described his son as a “traitor”.
Mr Litvinenko (43) died nearly three weeks after drinking tea laced with polonium-210 in London in November 2006.
Police concluded the fatal dose was probably consumed during a meeting with Dmitri Kovtun and Andrei Lugovoi at a hotel in central London.
British authorities later decided the pair – who deny involvement – should be prosecuted for murder.
On Tuesday it was finally confirmed Mr Kovtun would not give evidence after he withdrew at the 11th hour.
The video link to Moscow was briefly activated to show the inquiry's official sat next to an empty chair. He said Mr Kovtun had told him he would not be taking part.
Inquiry chairman Sir Robert Owen criticised Mr Kovtun and Russian authorities. He said: "This unhappy sequence of events drives me to the conclusion either that Mr Kovtun never in truth intended to give evidence and that this has been a charade.
“Alternatively, if he has at some stage been genuine in his expressed intention to give evidence, obstacles have been put in the way of his doing so.
In a statement given to the inquiry, Mr Kovtun claimed he had ended up in the bar at the Millennium Hotel with Mr Litvinenko and Mr Lugovoi “completely by chance”.
He said Mr Litvinenko had “flopped down” at their table before grabbing a teapot and pouring himself some tea.
“He gulped down two cups and then had a coughing fit. In the course of the conversation, he coughed constantly and wiped his mouth with a napkin.”
Mr Kovtun added he had the impression Mr Litvinenko had “mental health problems” and was “driven to despair”, adding: “He was prepared to do anything to achieve his financial goals.” He insisted he “knew nothing about Polonium 210 until Mr Litvinenko’s death”. – (PA)