Samantha Lewthwaite, 30, was gunned down by a Russian sniper two weeks ago, alleged Regnum news agency in Moscow.
One of Britain’s most wanted people, she was reported last month as fighting for the Islamic State’s terrorists in Syria.
The extraordinary Russian claim could not be verified at all today.
Lewthwaite was married to 7/7 suicide bomber Germaine Lindsay, 19, a Jamaican-born Yorkshireman, and allegedly left Britain in 2009 before being linked to atrocities in Africa.
White Widow has been reported as dead
She was last said to be in Syria training fighters for the Islamic State.
“One of the world’s most wanted terrorists Samantha Lewthwaite who fought as part of a Ukrainian battalion has been killed by a volunteer sniper,” said the news agency report.
“According to our information, the White Widow fought on the side of Ukrainian volunteer battalion Aidar as a sniper.
She was killed by “a volunteer sniper who came to fight at the Lugansk front from Russia”.
The report continued: “The Russian sniper who killed the British terrorist now has a price of nearly one million dollars on his head from Ukrainian special services.”
The extraordinary unconfirmed claim came amid a welter of propaganda from Moscow and Kiev over the Ukrainian crisis.
Reports last month claimed Lewthwaite – a former soldier’s daughter – joined Islamic State earlier this year and became one of the most powerful women in the network.
Once seen as a innocent home counties wife, Lewthwaite has since become one of the most notorious fugitives on the planet.
For the last three years British, American and Kenyan security services have been chasing her, with some reports suggesting she may have employed plastic surgery to evade capture.
Born in Northern Ireland and schooled in Aylesbury, she was not previously linked to Ukraine.
She was responsible for training a special team of female suicide bombers in Syria, it was alleged.
Samantha Leithwaite is presumed dead
Lewthwaite allegedly trained and mentored British converts who volunteered to fight for the IS.
She is so senior that she is known within the group as the “Special One” or the “Important Woman”, according to intelligence sources.
The 7/7 attack on the London transport network on 7 July 2005 killed 52 people.
Lewthwaite had been viewed as the innocent Home Counties’ wife of a suicide bomber but was later seen as “the world’s most dangerous female terrorist”.
In June 2012 two men and a white woman – believed to be her – wearing Islamic dress killed three and injured 25 at a beer garden in Mombasa after throwing grenades at foreigners watching England play Italy at Euro 2012.
She was also linked to another grenade attack which killed security guards in an upmarket restaurant.
The father of Lewthwaite this evening said he had not heard about his daughter’s death.
Builder Andrew Lewthwaite, 58, said: “I have heard nothing. No-one has told me anything about this. I’m not saying anything more about it.”