Baroness Glenys Kinnock died peacefully in her sleep on Sunday with her husband and former Labour Party leader Lord Kinnock by her side, her family said.
She served as a minister in the New Labour government and also represented Wales in the European Parliament as an MEP.
Her family, which includes Labour MP Stephen Kinnock and daughter Rachel, said they were “devastated” by her death.
Mr Kinnock, who serves as the shadow immigration minister, said he was “heartbroken” and called his mother “a truly formidable person in every single way”.
Labour leader Sir Keir Starmer called her a “true fighter” for the party as he paid tribute to her life and career.
The 79-year-old had been diagnosed with Alzheimer’s six years ago.
In a statement to PA news agency, her family said: “It is with the deepest sorrow that we announce the death of Glenys Kinnock.
“Glenys died peacefully in her sleep in the early hours of Sunday morning, at home in London.
“She was the beloved wife and life partner of Neil, the cherished mother of Steve and Rachel and an adored grandmother.
“Neil was with her in her final moments. They had been married for 56 years.
“A proud democratic socialist, she campaigned, in Britain and internationally, for justice and against poverty all her life.”
Baroness Kinnock served as an MEP for some 15 years before leaving Brussels in 2009 to take up a life peerage when then prime minister Gordon Brown appointed her minister for Europe.
In their statement, the family continued: “Passionate to the end about education, she was a valued and respected school teacher before she began her own political career, as a member of the European Parliament, then being made a peer in the House of Lords from where she served as minister for three of the great passions of her life, Europe, Africa and the UN.
“She was a great friend to many people and causes, and was truly loved.
“Glenys endured Alzheimer’s after being diagnosed in 2017 and, as long as she could, sustained her merriment and endless capacity for love, never complaining and with the innate courage with which she had confronted every challenge throughout her life.
“The family is of course devastated and would ask that their privacy be respected.
“Funeral details will be communicated in due course.”
Her son Mr Kinnock tweeted his own heartfelt tribute to his mother.
The Labour MP for Aberavon wrote: “Heartbroken that my Mum passed away peacefully in her sleep last night, after many years of Alzheimer’s.
“She was a beloved Mum & Nain who was adored by her family & friends.
“A truly formidable person in every single way, and with such a cheeky sense of humour! Rest in peace.”
Sir Keir said Baroness Kinnock was a “passionate lifelong campaigner for social justice at home and abroad” who had an “impressive political career” in her own right.
“Neil and Glenys had the most wonderful partnership, there for each other through thick and thin, with a love and commitment that was instantly obvious when you saw them together,” he said.
“As the family have detailed, in recent years that meant looking after Glenys as Alzheimer’s did its worst.
“But what we will all remember is Glenys as a true fighter for the Labour Party and the values of the labour movement, a pioneering woman, to whom we owe an enormous debt.
“My sincere condolences to Neil, Stephen, Rachel and all the family at this sad time.”
Former prime minister Sir Tony Blair said Baroness Kinnock was a “leader in her own right”, away from her role supporting her husband Neil, who was Opposition leader between 1983 and 1992.
Sir Tony, who led Labour between 1994 and 2007, described her as “incredibly smart, brave, determined and resolute in standing up for what she believed was right”.
Offering his condolences to Lord Kinnock’s family, he said: “Whether in fighting the cause of development, and the eradication of global poverty, social justice in Britain, equality for women or making the case for a European Union of weight and influence in the world, Glenys was passionate and persuasive.
“She was of course an enormous support to Neil but she was a leader in her own right.
“And as a couple, they were a joy to be near, full of fun, the life and soul of any gathering.
“Glenys will be mourned in many countries and corners of the earth.”
Mr Brown, Sir Tony’s successor in Downing Street, spoke of his and wife Sarah’s sadness following the death.
He said she had borne her lengthy illness with “great fortitude”.
“All who met Glenys admired her for her generosity, her warmth and her passionate support for the best of national and international causes,” said Mr Brown.
“She was a highly effective and popular minister for Europe in the last Labour government and I was delighted to have persuaded her to become a member of the government.”
Charity Dementia UK said that by publicly discussing their experiences of Alzheimer’s, the Kinnock family had helped raise awareness about the illness and the “challenge that a diagnosis can bring”.
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