At a cost of $190 million per vessel, Maersk said the total contract, including the option, is worth $5.7 billion, and the vessels will be delivered from 2013 to 2015.
The deal would be the biggest-ever order for South Korea's Daewoo, and help the world's No. 2 shipyard meet nearly all of its $6 billion target for 2011 shipbuilding orders.
The option for 20 additional ships was twice as big as earlier reports, and Daewoo shares leapt 4.8 percent in a soft Seoul market.
"Should we decide to go for all 30 vessels, it is going to be close to $6 billion, and that will make it the largest order ever seen in shipping," Maersk Line Chief Executive Eivind Kolding said at a news conference in London.
Kolding said in an interview with Reuters Insider television that he saw a 50 percent chance that Maersk would exercise an option for an additional 10 ships from Daewoo, but he called the option on the final 10 vessels a "long shot."
"Both options expire this year, so during the year we have to decide on both of them," Kolding told Reuters.
The new ships will add to Maersk Line's current fleet of more than 500 vessels and help it keep its industry-leading market position, Maersk said in a statement.
"You build ships for 25 years, so you need to take a long-term view, but even in the shorter term we are actually quite comfortable, and we will see growth of 5 to 8 percent also in the Asia-Europe trade," Kolding told Reuters.
Shares in A.P. Moller-Maersk, which said it would use the ships on Asia-Europe routes, closed down 1.6 percent.
The Triple-E class vessels, measuring 400 metres (1,312 feet) long, 59 metres wide and 73 metres tall, will be "the largest vessel of any type known to be in operation."
They will be 16 percent bigger in capacity than the biggest ships in Maersk's current fleet, the E-class vessels.
"This is new territory for the industry since it represents a big step up from around 14,000 TEU (twenty-food equivalent unit) vessels to 18,000 TEU," said Neil Dekker, a container shipping analst at Drewry consultants in London.
Kolding said the ships would cut unit costs by 26 percent.
"These ships are particularly planned for the Chinese market," Soren Karas, Maersk Line's head of South China business, told a news conference in Hong Kong. "There is no doubt China is the biggest part of the containerised transport market now, so a very important market for us."
ING analyst Axel Funhoff said the order was a bet on the future. "One thing is clear: If you can fill these ships, they will be more profitable than the average ship in the market."
Peter Sand, analyst with shipping association BIMCO, said the deal, Sand said, "gives Maersk a horsehead advantage at a time when competitors are challenged financially."
Growing Confidence
The deal reflects growing confidence in the global container shipping market, which has recovered from a disastrous 2009 amid the global financial crisis, and adds to a series of recent deals placed with the world's top shipyards.
"The handling of oversupply will stay a challenge in coming years as the fight for (vessel) employment and market share stays fierce," BIMCO's Sand said.
Daewoo's Korean rivals are also eyeing prospects for rising orders from global shippers amid a consumption pickup.
"Whether or not this order acts as a catalyst for similar size ships to be built for other operators remains to be seen, but no other companies have made any noises in public that this might be the case," Drewry's Dekker said.
Hyundai Heavy Industries received an order to build six large container vessels in February, reportedly worth around $800 million. (Reuters)
