Ken Livingstone, the former London Mayor and one-time figurehead of the Labour left, is suffering from Alzheimer's disease, his family has announced.
He is being "well cared for by his family and friends" as he lives a "private life" in retirement, they said in a statement issued to the PA news agency.
It comes after it was reported that the former MP withdrew a legal challenge to the Equality and Human Rights Commission's (EHRC) report into into Labour antisemitism.
The statement said: "In response to media enquiries the Livingstone family today announce that Ken Livingstone, ex-MP for Brent and former Mayor of London, has been diagnosed with and is living with Alzheimer's disease.
"Although a previously prominent public figure, Ken is now retired and lives a private life. He will no longer be available for any media interviews or requests and we will not be responding to any media questions or enquiries.
"Ken is being well cared for by his family and friends and we ask you for your understanding and to respect his privacy and that of his family."
While having largely retreated from public life in recent years, the 78-year-old was a prominent figure in London politics for more than four decades from the 1970s.
In his heyday, "Red Ken" was a thorn in the side both of Margaret Thatcher's Tories and New Labour under Sir Tony Blair.
He stood as an independent and became the first Mayor of London in May 2000 when then-prime minister Sir Tony created the powerful post.
In his second term, which he won as the official Labour candidate, he earned praise for the way he stood up for London after the July 2005 suicide bombings and helped win the 2012 Olympic Games for the capital.
Mr Livingstone lost City Hall in 2008 when he was defeated by an equally colourful opponent in Boris Johnson and a failed bid to return to office in 2012 marked the end of his electoral ambitions.
His time in the Labour party ended ignominiously as he quit in 2018 amid furious demands for his removal over allegations of antisemitism.
It came after a long-running row over his claims that Adolf Hitler had backed Zionism in the 1930s, which had originally seen him suspended from the organisation in 2016.
Mr Livingstone was singled out in a human rights watchdog report in 2020 into how Labour dealt with anti-Semitism claims, which said Jewish Labour Party members felt he had made comments that "had the effect of stirring up and fuelling hatred for Jews".
He had been seeking a High Court review of the report but on Tuesday it was reported by the Jewish Chronicle that the challenge had been withdrawn.