Many Labour Party delegates face staying outside Brighton and Hove during their conference this year because every hotel room is booked.
All delegates were able to stay in the city last year despite the event being a sell-out.
But Brighton and Hove City Council is now warning that they cannot be guaranteed accommodation this time round.
Letters are being sent to those who have not booked, advising them of the alternatives.
These include self-catering, staying elsewhere, including Worthing and Gatwick, and staying with local party members or council staff.
Amanda Shepherd, Brighton and Hove City Council's head of tourism, said: "I would like to think it has arisen because it is a very successful conference and we do look after Labour very well.
"At party conferences, in addition to delegates, there are large numbers that come to lobby as well as a lot of media.
"When that particular party is also in Government we get even more, including international and national media.
"We estimate in all there are about 20,000 visitors from the Sunday to the Thursday. This obviously has a large impact on the local economy - not only in terms of how much they spend here, but also because it places us on the map for national and international conferences."
This year's conference takes place between September 30 and October 4. Roger Marlowe, chairman of the local hoteliers' association, welcomed the sell-out and said it was a "real credit" to the council.
A Labour spokeswoman said: "The conference is the biggest in Europe so it is not surprising it is difficult for any city to accommodate it.
"There is always a huge demand for people wanting to come."
May 28 2001
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article