Baroness Castle was famous for fighting social causes
The former MP for Blackburn, who was a trail-blazer for women in politics, died peacefully in her sleep at her Buckinghamshire home on Friday afternoon, her family said.
Paying tribute Prime Minister Tony Blair said she was "a radical and independent spirit" and "an extraordinary pioneer for women in politics".
"Barbara Castle was one of the dominating figures of the Labour movement of the last 50 years," he said.
'Courageous'
She served in the Wilson governments of the 1960s and 70s as transport secretary and then as social services secretary.
A formidable champion of social causes, she was responsible for child benefit and the breathalyser test, and was an unstinting campaigner for pensioners' rights.
Mr Blair added: "She was courageous, determined, tireless and principled, she was never afraid to speak her mind or stand up for her beliefs.
"Britain has lost a great political figure and the Labour movement a heroine."
Former Labour prime minister Lord Callaghan paid tribute to "her tenacity and her sheer hard work".
"She fought to the end and she was always a fighter," he said.
And Baroness Boothroyd, the former Commons Speaker, who was once secretary to Baroness Castle said: "Barbara was a true friend and mentor."
She said although her death was not unexpected, Barbara Castle remained politically active to the last.
Baroness Boothroyd said she saw her friend in the House only a few weeks ago.
Outspoken
Labour MP Barbara Follett, who together with Lady Castle set up the Labour Women's Network, said her political passion would be missed.
She told BBC Radio 4's Today programme: "She once said to me that it was fury that keeps her going, and I identify with that, it's anger that keeps me going."
With her trademark flame-red hair Baroness Castle was the best known woman parliamentarian of her day - who some thought could have become the UK's first female prime minister.
She remained an active politician well into her 80s, keeping up a passionate commitment to the Labour movement.
Known for her outspoken comments she saw no reason to make concessions to Tony Blair and his government.
She was said to give them as much criticism as she did the Tories.
She launched a fervent attack on the Labour leadership at party conferences - once again taking up the cause of the elderly - for its attitude towards pensioners and the disabled.
She will also be remembered as being the architect of the Equal Pay Act.
As Transport Secretary she also introduced the 70mph speed limit.
But she will also be known for her failed bid to reform the trade unions.
Fighting motto
A widow of Baron Ted, she took the title of Baroness Castle of Blackburn after the constituency she represented in Westminster for more than 30 years.
But she always preferred to be addressed as Mrs Castle.
When her parliamentary career ended in 1974 she went on to become an MEP for 10 years and became a life peer in 1990.
Her memoirs, entitled Fighting All the Way, aptly described her personal motto.