Baroness Glenys Kinnock of Holyhead, the wife of former Labour leader Neil has died at the age of 79.
The family has shared tributes for the former MEP.
In a statement, 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.
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"A proud democratic socialist, she campaigned, in Britain and internationally, for justice and against poverty all her life."
Continuing their tributes to Lady Kinnock, her family said: "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.
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"The family is of course devastated and and would ask that their privacy be respected.
"Funeral details will be communicated in due course."
Glenys and Neil's son, Stephen Kinnock, an MP himself, also paid tribute on X formerly Twitter, writing: "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."
The current Labour Party leader Sir Keir Starmer has also paid tribute to Baroness Kinnock, who he called a "true fighter for the Labour Party", after her death was announced on Sunday.
He said: "On behalf of the whole Labour Party, I want to pay tribute to Glenys Kinnock on the sad news of her passing.
"Glenys was a passionate lifelong campaigner for social justice at home and abroad.
"She supported Neil through his leadership and went on to have an impressive political career of her own as a member of the European Parliament, in the House of Lords and as a minister in the last Labour government, focused on Europe and Africa.
"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.
"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 and Labour leader Sir Tony Blair said Baroness Kinnock's death would be "mourned in many countries and corners of the Earth".
In a statement, Sir Tony said: "Cherie and I are so sad the hear the news about Glenys. She was a huge figure in progressive politics for decades: incredibly smart, brave, determined and resolute in standing up for what she believed was right.
"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.
"In her last years, as Stephen and Rachel have written, she took her illness with the same steadfastness which had governed her life.
"Our deepest condolences to Neil, to Rachel and to Stephen and to all the wider Kinnock family. Glenys will be mourned in many countries and corners of the earth."
London mayor Sadiq Khan paid tribute and described Baroness Glenys Kinnock as "wonderful and decent".
The mayor said the news of her death was "heartbreaking" as he offered his condolences to Lady Kinnock's family.
"Heartbreaking news," Mr Khan wrote on X, formerly Twitter.
"Glenys was a towering figure in the Labour movement for decades and a thoroughly wonderful and decent human being.
"Neil, Rachel, Stephen and the entire family are in my thoughts and prayers."
Glenys served as a member of the European Parliament for some 15 years, before being appointed minister for Europe and receiving a life peerage at the same time.
It was sometimes said of her, before she became an MEP, that she played a large part in formulating Labour policy "over breakfast with her husband".
And even though she was regarded by some as even more of a left-wing firebrand than Neil, that story was always fiercely denied.
Baroness Kinnock had a wide field of interests but she was especially well-known for her work designed to alleviate poverty and starvation in Africa and other parts of the world.
Glenys Elizabeth Kinnock (nee Parry) was born on July 7, 1944, and was educated at Holyhead High School, Anglesey.
She graduated from University College, Cardiff, in education and history.
She met her future husband at university and they were married in 1967.
Baroness Kinnock subsequently worked as a teacher in secondary, primary, infant and nursery schools.
She became an MEP in 1994 and was a prominent member of several committees and for a period was Labour's spokeswoman on international development in the European Parliament.
But it was not all plain sailing.
In 2004, she was caught up in an expenses scandal in which she was one of scores of MEPs who allegedly signed in for the day at the European Parliament (to qualify for the £175 daily allowance) and then promptly left the building.
And in November, 2006 she was criticised in the press for taking what was described as "a junket" in Barbados to discuss world poverty issues.
She had the unenviable reputation as "the most travelled British MEP" and, along with her husband, also acquired the no less enviable title as Brussels' "very own Lord and Lady Expenses".
Baroness Kinnock was required to leave the European Parliament in 2009, when then prime minister Gordon Brown appointed her minister for Europe, following the resignation from that post of Caroline Flint.
Although, when her husband was ennobled some years earlier, she was entitled to be called Lady Kinnock, it was a title she never used.
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However, on her appointment as minister for Europe she became a peeress in her own right.
She is survived by her husband of 56 years, who was with her in her final moments, and her children Stephen, a Labour MP, and Rachel.