Rev Ian Paisley, the former Northern Ireland First MinisterPhotograph: Peter Muhly/AFP/Gett

The Reverend Ian Paisley has been rushed to hospital in Belfast suffering from an undisclosed illness.

While family members were stating it was for routine tests, reports were circulating that the former first minister was seriously ill. He has had several health setbacks in recent years and was once considered on his death bed before making a remarkable recovery.

Paisley, 87, was a huge figure in the Northern Ireland conflict from the very beginning, when his radical preaching was blamed for Protestant violence as far back as 1968.

He bitterly opposed any attempts at cross order reconciliation and brought down Northern Irish Prime Minister Captain Terence O'Neill, who tried to create better relations with the south. He was the first of many Unionist leaders to experience the wrath of Paisley and political oblivion as a result.

Though he never explicitly promoted violence, Paisley's Free Presbyterian church became a hot bed of Unionist opposition.

His anti-Catholic rhetoric even included a vicious attack on the pope at the European Parliament.

Though he bitterly opposed the Good Friday agreement, Paisley was eventually won over and in perhaps the greater watershed moment in Northern Ireland history, he agreed to serve with Sinn Fein leader Martin McGuinness. Paisley was elected First Minister of Northern Ireland, at the age of 81, on May 8, 2007 with McGuinness as deputy First Minister.

Their partnership ushered in a new era in Northern Irish politics, and he retired as First Minister of Northern Ireland in May 2008 to widespread accolades from all sides.

While the full nature of his illness is unknown, his age and declining health is a major cause for concern say media sources.