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In the Bannside by-election Dr Paisley gained a winning margin of more than 1,000 votes.
It was his second attempt to win the seat - at the last General Election he was 1,400 votes behind the then incumbent, Prime Minister Terence O'Neill.
This time the Bannside seat was vacant following Terence O'Neill's elevation to the peerage and his resignation as Northern Ireland's prime minister.
Lord O'Neill's attempts to reconcile Protestants and Catholics had long led to him being berated as a traitor to the unionist cause by right-wingers such as Dr Paisley.
An added boost for Dr Paisley was the victory in South Antrim of his fellow Protestant Unionist and Free Presbyterian Church minister, the Reverend William Beattie.
Their wins could spell problems for the reform programme currently under discussion in the Northern Ireland parliament at Stormont.
The province has been run by Protestant governments since the split from the Republic of Ireland in 1921
Its police force, the Royal Ulster Constabulary, is also overwhelmingly Protestant.
Direct rule fears
The current government, led by Major James Chichester-Clark, is negotiating a programme of social and policing reforms with Catholic groups.
Both Ian Paisley and William Beattie campaigned on a platform of opposition to a new deal for Catholics.
"The Unionist party led by Chichester-Clark has been deceiving and betraying us," Dr Paisley told cheering crowds after his victory was announced.
He denied his win was a victory for militancy in Ulster and said he would represent everyone in his constituency.
But some Stormont MPs had forecast a win by Dr Paisley would mark the beginning of the end of the Northern Ireland parliament which is already fragmented between moderates and right-wingers.
Many fear if the parliament at Stormont becomes any more fragmented direct rule from London may be imposed.
In June 1970 Ian Paisley entered the House of Commons after winning North Antrim.
He has fought a long-running feud with the Ulster Unionist party - in 1971 he formed the rival Democratic Unionist Party and has been its leader ever since.
He has been opposed to every political initiative to bring power sharing to Northern Ireland.
He opposed the Sunningdale Agreement in 1973 and was deeply involved in the Ulster Workers' Strike which brought down the power-sharing administration in 1974.
He also resisted the Anglo-Irish Agreement in 1985 and the agreement reached at Stormont on Good Friday in 1998.
He opposed the agreement but sat in the Northern Ireland assembly, until it was suspended in 2002.
In elections to the Assembly in November 2003, the DUP and Sinn Fein emerged as the largest parties.
Finally at an historic meeting in March 2007, Ian Paisley met Sinn Fein leader Gerry Adams and agreed a power-sharing deal to begin in May 2007.
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