Martin McGuinness has died aged 66.
The Northern Ireland politician, who served as deputy first minister in the province for almost a decade, stepped down from office earlier this year after being diagnosed with a rare genetic disease.
McGuinness, previously a member of the Irish Republican Army (IRA) before joining Sinn Fein, was instrumental in the Northern Irish peace process.
He was Sinn Fein's chief negotiator in the Good Friday Agreement negotiations, and spent 16 years as an MP, although he never took his seat.
He passed away earlier this morning in Derry.
"It is with deep regret and sadness that we have learnt of the death of our friend and comrade Martin McGuinness who passed away in Derry during the night," Sinn Fein said in a statement today.
Prime Minister Theresa May said: "First and foremost, my thoughts are with the family of Martin McGuinness at this sad time.
"While I can never condone the path he took in the earlier part of his life, Martin McGuinness ultimately played a defining role in leading the Republican movement away from violence. In doing so, he made an essential and historic contribution to the extraordinary journey of Northern Ireland from conflict to peace."
She added: "While we certainly didn’t always see eye-to-eye even in later years, as deputy First Minister for nearly a decade he was one of the pioneers of implementing cross-community power sharing in Northern Ireland.
"He understood both its fragility and its precious significance and played a vital part in helping to find a way through many difficult moments."
At the heart of it all was his profound optimism for the future of Northern Ireland – and I believe we should all hold fast to that optimism today.
Labour leader Jeremy Corbyn said: "I was very sad to hear of the death of Martin McGuinness this morning, and I would like to send my thoughts and condolences to his wife Bernie and his family."
Martin McGuinness played an immeasurable role in bringing about peace in Ireland, after years as a key protagonist in the tragedy of the conflict.
"Martin played an absolutely crucial role in bringing about the Good Friday Agreement and a peace process which, despite difficulties, remains an example throughout the world of what can be achieved when the will is there," Corbyn added.
"As we reflect on his role, the past 20 years have shown us that if there is leadership and the will on all sides, we can achieve change."
James Brokenshire MP, the secretary of state for Northern Ireland, said: "I want to extend my sympathy and sincere condolences to the family of Martin McGuinness at this difficult time.
"Martin’s personal journey and the clear influence he had on others in the Republican movement were instrumental in shaping political institutions in Northern Ireland founded on exclusively peaceful and democratic means."
Brokenshire added: "While not forgetting the past, no-one can doubt the essential role he played in helping to secure the power sharing arrangements and political progress in Northern Ireland. Martin’s commitment to reconciliation and understanding across communities was a significant factor.
"Whilst passionate and robust in his politics, on a personal level I always found Martin to be thoughtful and reflective and appreciated the personal consideration he showed. The importance of family and his home in Derry shone through.
"Martin will be remembered for his contribution to politics in Northern Ireland and particularly during his near 10 years as deputy first minister."