Liberal Democrat peer Baroness Diana Maddock has died at the age of 75, the party confirmed.
Baroness Maddock, who was elected as MP for Christchurch in 1993, had served as Lib Dem president between 1998 and 2000.
Appointed to the Lords in 1997, she most recently acted as deputy chief whip for the party's group in the upper chamber.
Acting leader Ed Davey said: “It is with great sadness that we have learned of the death of Diana Maddock.
“Diana has been a great friend, support and inspiration to me over many years and I will miss her terribly.
“My thoughts and prayers are with her family at this time.”
She had been treated successfully for breast cancer twice, continuing her work in the House of Lords throughout, but she was informed in the spring that the cancer had returned.
The party's group at the House of Lords wrote on Twitter: "We are so very sad to tell you that our dear friend and colleague Diana Maddock has died.
"She will be tremendously missed by us all.
"Our thoughts and love are with Alan and all her family at this time."
Former leader Tim Farron wrote: "So sad. Diana was steely, sensitive, wise and warm.
"Her win in Christchurch was instrumental to the party’s huge leap forward under Paddy Ashdown.
"Lots of love to Alan and all the family."
Lib Dem leadership contender Layla Moran wrote: "What I remember most about Diana was her warmth and her smile.
"Once, early in my career she took me aside after a meeting just to chat and give an encouraging word about the work I was doing.
"Such a kind person. She will be sorely missed."
Lib Dem peer Chris Rennard posted: "We will greatly miss a friend and colleague with who I was happy to work as candidate, MP, President and Peer."
She is survived by her two daughters from her first marriage, Anna and Becki, along with husband and fellow peer Lord Alan Beith, formerly the MP for Berwick-upon-Tweed for more than 40 years.
