Cardinal Thomas Winning, leader of Scotland's Roman Catholic community, died suddenly today, a church spokesman said.
The cardinal, 76, had been discharged from hospital on Friday where he had been admitted June 8 following a heart attack.
He is believed to have suffered another heart attack at his home in Glasgow.
The Archbishop of Westminister, Cormac Murphy-O'Connor, leader of the Roman Catholic Church in England and Wales, said in a statement he was "deeply saddened."
He described the cardinal as "an outstanding leader of the church in Scotland and beyond. His humour, dedication, utter loyalty and unstinting defence of the Catholic church will long be remembered."
Helen Liddell, the British minister responsible for Scotland, said Dr Winning was "a man of great vision and immense social conscience. He was particularly concerned by the problems of poverty, and worked tirelessly to help those in most need."A memorial mass will be held in Westminster Cathedral in London tomorrow.
Dr Winning was well-known in the Vatican's corridors of power. He attracted international media attention by offering money and support to pregnant women as an alternative to abortion.
Although politically left-of-centre, he was unflinchingly conservative in church matters. He spoke out against homosexuality and did not offer any support for campaigners for married priests.
He took a high-profile stance against the 1991 Gulf War in public and lobbied against it at the highest levels.
When he was made a cardinal in 1994 his personal popularity among Scotland's Catholic community made Vatican insiders whisper his name as a possible successor to Pope John Paul II.
Less than a year later he had to deal with the most difficult crisis to face the Catholic Church in Scotland this century - when one of its bishops, Roddy Wright, ran off with a woman and admitted he had fathered a child by another woman.
In 1997 Dr Winning accused Prime Minister Tony Blair of gagging members of parliament from his Labour party who were opposed to abortion. Later that year he launched a scheme dubbed "cash-for-babies" where women were offered counselling and financial support as an alternative to abortion.
It was to prove controversial, with pro-abortion campaigners decrying a decision to offer a 12-year-old girl money to stop her from having an abortion.
The death of his English counterpart Cardinal Basil Hume in 1999 made Dr Winning Britain's most senior Catholic.
But with characteristic modesty he did little to assume the mantle of leader to the whole British flock and made clear to his inner circle he had no ambition to become pope.
AFP