Tournament chief executive Roger Pickering acknowledged that the Ireland-England match on March 24 was already in jeopardy.
'We will be looking at contingency plans in the likely event of the Ireland- England game not going ahead,' he said. 'Tickets will be valid for the match or matches when they are re-arranged.'
England manager Clive Woodward, determined to land the Slam this year after falling at the last fence for the last two seasons, now realises that the major threat to achieving it comes from something over which he has no control.
'There are far more important matters at stake than international rugby.
'Nobody wants to see the tournament disrupted but the farmers' plight is far greater than a game of rugby. We understand what's going on.'
Wales manager David Pickering said: 'We fully appreciate the seriousness of the outbreak and our heart goes out to those affected by it. We were desperate for the Irish match to go ahead but common-sense had to prevail.'
Wales and Ireland are considering April 29, one of the very few free weekends at the Millennium Stadium given their staging of all English soccer's traditional end-of-season showpiece matches.
An alternative date for England's match in Ireland would be virtually impossible without running head-long into the final stages of the European Cup and English Premiership play-offs.
The Wales-Ireland postponement is only the second Test in the British Isles to be hit by foot-and-mouth and the first for 34 years.
Ireland called off their match against New Zealand in Dublin on December 16, 1967 for exactly the same reasons behind yesterday's decision, an outbreak of the disease in mainland Britain.
Apart from robbing the All Blacks of a Grand Slam, it forced them to burn their kit before leaving London at the end of the tour.
Five years earlier a smallpox epidemic in South Wales forced Wales to postpone their home fixture with Ireland until the following November, an option which is no longer available given the crowded list of pre-Christmas internationals.
The Lions' scheduled departure for Australia on June 1 eliminates any question of the tournament being completed in the last fortnight of the season.
Unless the current outbreak can be swiftly contained, the Six Nations may be resigned to an incomplete tournament which will raise contractual issues with television and sponsors.
