Already on a years-long growth spurt, MSC Cruises is adding a new category to its ship-building boom: luxury.

The privately owned European operator announced Thursday it has agreed to order four “ultra-luxury” cruise ships for more than 2 billion euros, or $2.3 billion. The vessels will have capacity for 1,000 passengers — far smaller than the rest of MSC’s fleet — and encompass 64,000 gross tons.

The first will start sailing in spring of 2023, with the next three joining at the rate of one a year for the next three years. Italian shipyard Fincantieri is building the ships.

Details were not available yet about the new class of ships, but executives highlighted the MSC Yacht Club, a ship-within-a-ship concept, as an inspiration. Eight ships have the private enclave, which MSC describes as “the most affordable luxury in cruising.”

“It is off the back of the great success of our ship-within-a-ship luxury concept that our guests asked us to enter into the ultra-luxury segment, as a natural evolution of the MSC Yacht Club,” MSC Cruises Executive Chairman Pierfrancesco Vago said in a statement. “These ships will be able to offer unique itineraries, thanks to their size, and the guest services will be taken to another level, including our personalized MSC Yacht Club butler service, available 24/7.”

By 2027, MSC Cruises will have a fleet of 29 ships, nearly double its current number thanks to an investment plan that tops $15 billion. Seventeen of those ships will have arrived between 2017 and 2027.

Headquartered in Switzerland, the line is a major player in Europe, the Gulf, South Africa, and South America. MSC has been making a push into North America as it grows, but its presence in the region is still relatively small.

Luxury lines including Crystal Cruises, Silversea, Regent Seven Seas Cruises, and Seabourn have added ships in recent years or have new orders on the books. And new entrants such at Ritz-Carlton Yacht Collection have also taken aim at the market.

That growth comes as the overall cruise industry sees healthy growth. An estimated 26.7 million people took a cruise in 2017, up 6 percent from the previous year, with the number expected to climb to 28 million this year.

Photo Credit: A butler is shown in the MSC Yacht Club, a small high-end enclave within a larger MSC Cruises ship. The privately held company is building four "ultra-luxury" ships, it announced Thursday. MSC Cruises