Former Welsh Secretary Dame Cheryl Gillan has died at the age of 68 after a long illness.
Prime Minister Boris Johnson paid tribute to the Conservative MP for Chesham and Amersham, saying she will be "sorely missed".
He described Dame Cheryl as "a great servant" and "always full of wise advice and good humour".
Dame Cheryl had served as the MP for the Buckinghamshire seat since she was first elected in 1992.
Co-chairwoman of the Conservative Party, Amanda Milling, said the noted anti-HS2 campaigner had made a "huge contribution to public life".
Ms Milling said: "It was incredibly sad to hear that Dame Cheryl Gillan MP passed away at the weekend.
"Cheryl had been ill for some time, but battled her illness with great stoicism and grace.
"Cheryl was a dedicated parliamentarian for many decades, serving in the cabinet and she made a huge contribution to public life and our party.
"Our thoughts and prayers are with Cheryl's family and friends."
Former marketing executive Dame Cheryl was acting joint chair of the 1922 Committee of Tory backbench MPs from May to September 2019.
That year, along with Charles Walker MP, she helped preside over the Conservative leadership contest to elect a successor to former prime minister Theresa May - a contest Mr Johnson won.
Born in Llandaff in Cardiff, Dame Cheryl was Welsh Secretary between 2010 and 2012.
Welsh Conservative Senedd leader, Andrew RT Davies said Dame Cheryl was a "model of public service and was respected across the political spectrum".
"She was incredibly proud of her Welsh roots and served with distinction as secretary of state," he said.
"Cheryl was a good friend to the Welsh Conservatives and will be sorely missed."
I'm heartbroken to hear the desperately sad news that my colleague Cheryl Gillan has passed away. She had been a great source of strength as we went through chemo together. When she told me she was dying I told her all the nice things I would say so she knew...1/— Tracey Crouch (@tracey_crouch) April 5, 2021
Conservative MP for Chatham and Aylesford, Tracey Crouch, who revealed her own breast cancer diagnosis in June, said in a series of messages on social media that she was "heartbroken to hear the desperately sad news" and felt "blessed" to have known Dame Cheryl.
"She had been a great source of strength as we went through chemo together," she wrote.
"When she told me she was dying I told her all the nice things I would say, so she knew she was an incredibly inspirational woman in politics who fought for the things she believed in.
"She was always kind and helpful to us newbies. She had oodles of wisdom from her years as an MP which she shared willingly. I shall miss her greatly."
Labour leader Sir Keir Starmer said he was "very sad to hear that Dame Cheryl Gillan has passed away".
He added on Twitter that he had worked with Dame Cheryl "on a number of issues" and that she was "respected across the House and a great champion for her constituents".
"My thoughts are with her family and friends at this sad time."
Speaker of the House of Commons, Sir Lindsay Hoyle, said Dame Cheryl's death would leave "a big hole in all our lives".
He said she "was one of the most popular, friendly and kindly people" in the Commons.
"An assiduous attendee of debates, Dame Cheryl never passed up an opportunity to speak against the proposed high-speed rail link HS2," he added.
BBC political correspondent Chris Mason added that, in time, there would be a by-election in the seat she represented for nearly three decades.
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