McGuinness made history by entering a government with his once bitter foe Ian Paisley of the DUP. The decision to share power was a key part of the peace process in Northern Ireland. (AFP)
Northern Ireland's former deputy first minister and one-time IRA commander Martin McGuinness has died aged 66, his Irish nationalist party Sinn Fein said on Tuesday.
McGuinness had resigned from politics in January, citing a serious illness and a breakdown in relations with the rival Democratic Unionist Party.
"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.
The BBC said he had died of a rare heart condition.
Sinn Fein refused to appoint a replacement for McGuinness in January due to a row with the DUP, its partner in a power-sharing government set up to bring peace to the province.
That triggered local elections in which Sinn Fein made major gains against the pro-British DUP and the two parties are currently deadlocked, meaning that the British government could impose direct rule of Northern Ireland from London.
Ten years ago in May McGuinness had made history by entering a government with his once bitter foe, Ian Paisley of the DUP.
The decision to share power was a key part of the peace process in Northern Ireland, which endured three decades of violence in which more than 3,500 people died.
McGuinness was a commander in the IRA paramilitary group that fought for the province to leave Britain and join the Republic of Ireland to the south.
In a memorable gesture that would once have been unimaginable, he shook hands with Queen Elizabeth II during her visit to Belfast in 2012.
Sinn Fein President Gerry Adams called his former colleague a "passionate republican who worked tirelessly for peace and reconciliation and for the re-unification of his country.
"Throughout his life Martin showed great determination, dignity and humility and it was no different during his short illness," he said in a statement.
McGuinness was born in 1950 in Derry, on the province's northern coast, and grew up in the city when it was riven by sectarian bloodshed.
He became involved in the civil rights movement as a teenager and rapidly rose through the ranks of the IRA.
He was second in command at the time of Bloody Sunday in 1972, when British soldiers shot 26 unarmed republican protesters in Derry, killing 13 of them.
He served prison time for IRA-related activities, but later became a peacemaker as he negotiated a deal with loyalists and the British government.
McGuinness became Northern Ireland's deputy first minister in 2007 working alongside former adversary Paisley, who was first minister.
Paisley's son Kyle tweeted that he was "very sorry to hear about the passing of Martin McGuinness. Look back with pleasure on the remarkable year he and my father spent in office together."
More recently, he served alongside first ministers Peter Robinson and then Arlene Foster until he quit in January over an energy policy scandal.
Northern Ireland's Martin McGuinness, who died on Tuesday aged 66, is a former paramilitary who became a key behind-the-scenes negotiator for the peace process.
McGuinness, who resigned from politics in January due to ill-health and a breakdown in relations with the rival Democratic Unionist Party (DUP), died during the night, his Irish nationalist party Sinn Fein said.
He was a commander of the Irish Republican Army (IRA) during its bloody campaign against British rule of the province, in which 3,500 people died in three decades of violence.
But the senior Sinn Fein member also helped negotiate the landmark Good Friday Agreement in 1998 for sharing power with one-time bitter foes the DUP.
The man described as a ruthless commander became a statesman who rubbed shoulders with prime ministers and presidents, as well as Queen Elizabeth II.
He said recently he hoped in the future to be "an ambassador for peace, unity and reconciliation".
"Reconciliation, I have always believed, is the next vital stage of the peace process," he said.
"My record of reaching out, whether it be to Queen Elizabeth -- and her record of reaching out to me on several occasions -- my visits to the Somme, to Flanders field, have not been reciprocated by the DUP and that is a particular disappointment to me."
In his position as deputy first minister, McGuinness shook hands with the queen, whose authority in Northern Ireland he does not recognise, during her visit there in 2012.
'Goodbye and God bless'
He wished the monarch well in Irish, in a phrase that translates as: "Goodbye and God bless".
McGuinness was born in 1950 in Londonderry in Northern Ireland, and as a teenager became involved in the civil rights movement.
He joined socialists Sinn Fein, now the main Catholic republican party, in 1970.
He became a member of the IRA and had risen to its second-in-command in Derry by the time of "Bloody Sunday", the notorious day on January 30, 1972, when British soldier shot dead 13 unarmed civil rights protesters.
Although McGuinness escaped detention by the British in Northern Ireland, he was jailed in the Republic of Ireland in 1973 after being caught with 250 pounds (113 kilogrammes) of explosives and nearly 5,000 rounds of ammunition in a car.
At his trial, where he received a six-month sentence, he proudly declared his membership of the IRA, saying: "We have fought against the killing of our people. I am a member of Oglaigh na hEireann (the IRA) and very, very proud of it."
Years later in May 2001, after McGuinness became a politician, he spoke of his membership of the IRA with a level of honesty that few of his Sinn Fein colleagues have matched and which has removed some of the toxicity from his past.
McGuinness moved into politics relatively early, becoming one of five Sinn Fein members elected to the short-lived Northern Ireland assembly in 1982.
He was involved in secret talks with British officials between 1990 and 1993, and after the IRA ceasefire of 1994, became Sinn Fein's chief negotiator in the painstaking talks that led to the Good Friday peace accords.
When a power-sharing government was established in 2007, McGuinness was appointed deputy first minister, working closely with First Minister Ian Paisley, the firebrand Protestant preacher who led the staunchly pro-British DUP.
The two men surprised many by striking up a warm relationship, and McGuinness continued good working ties with Paisley's successor, Peter Robinson.
But he fell out with Robinson's successor Arlene Foster, leading to his resignation in January.
McGuinness had said his ill-health, which required a period of absence from his role, was not the reason he resigned but said he was quitting politics to try to recover.
Sinn Fein refused to appoint a replacement for McGuinness due to a row with the DUP.
That triggered local elections in which Sinn Fein made major gains against the DUP and the two parties are currently deadlocked, meaning that the British government could impose direct rule of Northern Ireland from London.
McGuinness enjoys fly-fishing, and his condemnation of violence in his soft Derry accent have earned him death threats from the paramilitaries he once led.
The father of four has repeatedly condemned attacks by dissident republicans opposed to the peace process, and his outrage at the murder of two British soldiers in Northern Ireland in 2009 was seen as a bold step.