Royal Caribbean Cruises Ltd. said Friday that its planned merger with P&O Princess Cruises PLC is dead unless P&O shareholders approve it next week.

The ultimatum came as Carnival Corp. claimed significant interest among Princess shareholders in considering its rival bid.

Carnival, which has sweetened its offer three times, said it has 29 percent support among Princess for delaying the vote on Royal Caribbean's offer indefinitely.

Princess and Royal Caribbean announced their plans to merge in late November. Carnival entered the bidding in mid-December. A combined Princess-Royal Caribbean would replace Carnival as the industry leader by combining the second- and third-largest cruise companies.

Royal Caribbean chairman Richard Fain said if the merger isn't approved by P&O Thursday, there will be no deal.

"I want to be very clear," he said.

Earlier in the day, P&O said Carnival was offering "a realistic price" for the first time with its fourth hostile takeover offer of $5.4 billion in cash and stock.

But P&O chief executive Peter Ratcliffe again endorsed the merger with Royal Caribbean, which would create a $6 billion business, of which Princess would own just over half.

Ratcliffe expressed concern that Royal Caribbean might pull out and questioned the motivations of Carnival and shareholders interested in cashing out.

P&O called Carnival "indifferent" about a takeover and breaking up the merger because it would stay on top either way.

"Carnival has nothing at stake here and therefore has no downside," Ratcliffe said. "The board does not believe that there is the commitment from Carnival to deliver" on its bid.

Royal Caribbean has dismissed Carnival's offers as empty and simply intended to scuttle the merger vote. The two are crosstown rivals in Miami.

Carnival, whose top executives have been personally wooing P&O shareholders for more than a week, wants them to indefinitely delay the merger vote.

Carnival chairman Micky Arison, who has set up camp on P&O's home turf in London, said, "We are delighted with the growing level of support for an adjournment."

Royal Caribbean and P&O argue a deal with Carnival faces more regulatory hurdles than the planned merger because Carnival does more business in Europe. Carnival says the risks are nearly equal.

Shares of Carnival closed up 95 cents Friday at $26.62, while Royal Caribbean shares fell 5 cents to $17.25, both on the New York Stock Exchange. Princess' U.S.-traded shares fell 26 cents to $23.35 on the NYSE.