The Tory MP, 53, resigned as security minister in July after telling Boris Johnson that his recovery from lung cancer surgery in February was taking “longer than expected”.
On Tuesday he said he was “keeping upbeat” over his condition but “needs space to focus on treatment”.
Mr Brokenshire, who served as NI Secretary of State from 2016 to 2018, said: “Regrettably my cancer has progressed. Am starting a new line of treatment this week & receiving great care from the team at @GSTTnhs.
“Unwelcome news but keeping upbeat. Cathy & the kids have been amazing & appreciate all the kind messages. Now need space to focus on treatment.”
Prime Minister Boris Johnson, responding to Mr Brokenshire’s tweet, said: “So sorry to hear James.
“Our thoughts are with you and your family, and we hope you recover soon.”
A long list of people replied via Twitter to wish him well with the treatment.
Labour leader Keir Starmer said: “This is very sad news James but glad to hear you’re in good hands. Wishing you a speedy recovery and my thoughts with you and your family.”
Claire Sugden MLA sent her “very best wishes” to him and his family adding: “Hoping for a speedy recovery”.
UUP deputy leader Robbie Butler MLA simply said simply, “Praying for ya James” while DUP MP Carla Lockhart MP said it was “Terrible news” but that he is in good hands. “Trusting you make a speedy recovery and are back in Parliament soon,” she added. Another former Secretary of State, Julian Smith, also said he was “very sorry” to hear the news.