By Julia Wheeler
BBC Gulf correspondent
Saudi Arabia's de-facto ruler is to boycott a Gulf summit on Monday after disagreements with the host, Bahrain, over its free trade deal with the US.
Crown Prince Abdullah: In effect Saudi ruler since 1995
No official reason has been given for Crown Prince Abdullah bin Abdul Aziz not going to Manama, but reports say Saudi Arabia is unhappy about the deal.
Saudi Arabia says the bilateral deal is against conventions of the six-member Gulf Co-operation Council.
But Bahrain says it has acted within the Council's rules.
Gulf officials have been trying to play down the dispute, saying there are far more important issues for the two-day GCC summit.
Now that Crown Prince Abdullah has made his feelings clear by staying away though, it seems certain the issue will overshadow the meeting.
The free trade agreement was signed in September and is due to be ratified by both sides in the New Year.
The issue is its bilateral nature.
Saudi Arabia accuses Bahrain of acting outside the interests of the economic and political group. Its concern is that such a move hampers efforts to bring together the Gulf countries' economies.
Despite speculation it was going to back down in the face of its larger, powerful neighbour, Bahraini officials have been quoted as saying they will go ahead.
Saudi Arabia is also at odds with Qatar over al-Jazeera TV's controversial coverage of the region.
The kingdom is the most conservative Gulf state. There is concern that if its senior leaders stay away from the summit, they may be further alienated from the group.