The former deputy first minister at Stormont will not stand for re-election
Tributes are being paid to Sinn Féin's Martin McGuinness, who has announced he is quitting frontline politics.
Mr McGuinness says he is not seeking re-election to the Stormont Assembly and departing early on health grounds.
Sinn Féin leader Gerry Adams has thanked his friend, who he first met nearly 50 years ago.
"I want to express my heartfelt thanks to Martin McGuinness. He and I first met over 45 years ago behind the barricades in Free Derry and we have been friends and comrades since that time.
"I also want to thank Bernie and the entire McGuinness family for the support they have given to Martin over many years and for allowing him to become the leader, the patriot, the peacemaker and poet that he is.
"We were all shocked when we saw his appearance recently. Thank God he is looking a lot better since then and responding well to the treatment he is receiving.
"However, he does need to take time out to get better for himself, for his family and for our struggle.
"As we now know he won't be standing in the assembly election. That means we have to ensure that the election works for all of the people of the North and that we succeed in building on the progress that has been made since the Good Friday Agreement.
"Martin has said he wants to come back and be part of the process to end partition, build reconciliation, unite our people and achieve Irish unity.
"So on behalf of Sinn Féin and republicans everywhere I want to send him our best wishes. Give him the space to get better and increase our efforts so that when he returns the process of change has advanced."
Taoiseach Enda Kenny said: "I am sorry to learn that Martin McGuinness has decided not to contest the forthcoming election due to his poor health.
"While Martin and I may not always have seen eye-to-eye on every issue, I readily acknowledge the remarkable political journey that he has undertaken.
"I know that Martin remains firmly committed to delivering a peaceful and prosperous society for all of the people of Northern Ireland.
"He was one of the key architects of the Good Friday Agreement, and a tireless and committed champion of the peace process.
"I have appreciated working closely with Martin in recent years, including in particular in the work of the North-South Ministerial Council.
"I wish Martin and his family well for the future and I hope that his health will now be his absolute priority in the time ahead."
British Prime Minister Theresa May said: "Martin McGuinness served the people of Northern Ireland as deputy First Minister for nearly a decade.
"We recognise his work over many years securing a number of significant political agreements.
"He played a key role in moving the Republican movement towards a position of using peaceful and democratic means.
"I want to send him best wishes for his retirement.
"We will all continue to work to make sure that the people of Northern Ireland are able to live freely and in peace."
The former Stormont First Minister Arlene Foster also paid tribute to her former deputy.
"Martin McGuinness and I have had our political differences and we come from very different angles of vision.
"When I was elected as First Minister in January 2016 I indicated that alongside him I was determined to work with him for the betterment of all the people of Northern Ireland in order to build a better future for everyone.
"While the current political situation is not what any of us would wish and there is much work to be done to return stable government to Northern Ireland, I nonetheless value the good things achieved by the outgoing Executive and the contribution made by Mr McGuinness to it.
"As Deputy First Minister for almost a decade Martin McGuinness has been a major figure at Stormont.
"While never forgetting the past I believe the work at Stormont provided the foundations for our relative peace today."
"Despite all that has happened I wish Martin McGuinness a speedy recovery and that he and his wife are able to enjoy time with their family away from the relentless focus of public life."
Foreign Affairs Minister Charlie Flanagan said: "The decision of Martin McGuinness not to contest the forthcoming election to the Northern Ireland Assembly will be received by many with an equal measure of understanding and disappointment.
"While those of us who have worked directly with Martin will wish him and his family well for the future, we will also miss his positive contribution to political discourse on this island.
"During his tenures as Minister for Education and deputy First Minister, Martin was unstinting in his personal efforts to secure the stability of the power-sharing institutions in Northern Ireland and to advance reconciliation between the unionist and nationalist communities.
"As the holder of a joint office, he fully recognised that his duty was to represent all of the people of Northern Ireland. Through word and deed, Martin sought to reach out to those who – for understandable reasons – would have regarded his past with fear, anger and suspicion.
"Martin and I come from very different, and indeed mutually critical, political traditions. Yet, in the two and a half years that I worked directly with Martin, I experienced a political leader who was determined to make the future of Northern Ireland, and its people, so much better than its past."
While Fianna Fáil leader Micheál Martin added: "I would like on my own behalf, and on behalf of the Fianna Fáil party, to extend our best wishes to Martin McGuinness on his retirement and our hope that he is able to overcome what is a very serious health situation.
"I will not pretend to agree with many of the positions which he has taken over the last forty years, but I believe he has sought to be a constructive force in trying to make the post-Good Friday Agreement institutions work."