Mr Ensign, 51, a staunch conservative with a record of strong family-values stances, vowed to remain in office after describing the affair as 'absolutely the worst thing I've ever done in my life' at a news conference.

He said he and his wife, Darlene, have reconciled thanks to marital counselling and that his marriage is 'stronger than ever.' Darlene Ensign, with whom the senator has three children, also issued a statement insisting their marriage was sound but was not present at her husband's news conference.

An aide in Mr Ensign's office told the Washington Post that the affair took place from December 2007 to August 2008 and involved a campaign staffer who is married to a member of Mr Ensign's staff.

'I will not mention any names but the woman who I was involved with and her husband were close friends and both of them worked for me,' Mr Ensign said.

'Our families were close. That closeness put me into situations which led to my inappropriate behaviour. We caused deep pain to both families and for that I am sorry.'

Mr Ensign said earlier this month that he was not planning a 2012 presidential bid but wouldn't entirely rule it out.

Last year, Mr Ensign was rated the most conservative US senator by the right-wing National Journal and led the unsuccessful charge in 2007 to force Idaho Senator Larry Craig to resign from office after Mr Craig was caught in a gay sex sting operation.

In the past year, Mr Ensign has raised his national profile with numerous national TV appearances and is chairman of the Republican Policy Committee. Nevada political operatives said it remains to be seen how this impacts Mr Ensign's career. He is not due to run for re-election until 2012.

'There's some irony that the ones who stake out the highest moral positions are the ones who literally get caught with their pants down,' said Gary Gray, a Las Vegas-based Democratic political campaign consultant. -- AFP