Former US presidential candidate John Kerry has been forced to pull plans to visit the North this weekend.
The Democratic Massachusetts senator, who challenged President George Bush unsuccessfully in the 2004 election, was due to visit the University of Ulster in Derry to deliver a prestigious lecture on security in the 21st century.
He was also expected to meet a number of the North's politicians during his visit to the Magee Campus in Derry.
University authorities have been told to postpone his lecture because of pressing business in the US Senate in Washington.
Mr Kerry would have joined a distinguished list of guest speakers who have delivered lectures under a series commemorating veteran Democratic politician Tip O’Neill.
They include former US president Bill Clinton, United Nations secretary general Kofi Annan and ex-European Commission president Romano Prodi.