Two of Northern Ireland's political parties have begun a courtroom showdown in a row over one of the seats won in the general election.

Ulster Unionists claimed on Monday they have overwhelming evidence of electoral fraud and malpractice in a key Northern Ireland constituency on June 7.

Members of the Ulster Unionists and Sinn Fein faced each other in a Belfast High Court over the result of the Fermanagh and South Tyrone seat which Sinn Fein MP Michelle Gildernew won by just 54 votes.

UUP chairman James Cooper, who contested the seat, said as the case progressed he will present a dossier of what he will claim were "voting irregularities" that took place at a polling station. Cooper has claimed the polling station at St Martin's Primary School in the border village of Garrison stayed open after the 10pm deadline.

The seat was previously held by UUP veteran Ken Maginnis until he stood down at the last election.Cooper, a solicitor, defended his legal action saying he had "a duty to society and to unionism in Northern Ireland to make sure the electoral process retains some credibility".

"We believe, as the case develops, that we will illustrate and show to the people of Northern Ireland the contempt and disdain with which Sinn Fein approach the rules of democracy that should prevail in every election," he said.

Cooper's counsel, Declan Morgan QC, told the court that presiding officer John McGovern had been intimidated by Sinn Fein electoral workers to keep the poll open unlawfully after 10pm."The evidence demonstrates that there was a complete breakdown of order within the polling station at about the time of its close and that the extent of this threat and intimidation which was felt by those within the polling room was such that the presiding officer decided to keep the poll open, despite having the clearest advice that to do so was unlawful,'' he said.

Gildernew, who is also a Stormont Assembly member, dismissed the challenge as "a case of sour grapes".

"The UUP is clearly clutching at straws and they should think long and hard before rejecting democracy in such a fundamental way. Just because the UUP is unhappy with the outcome of the process does not mean that they should seek to overturn democracy in action,'' she said.

The case, which is expected to last for at least three days, was adjourned on Monday.