Sea-Land Service Inc. will sign the world's biggest container-handling contract on April 14.

The deal with Europe Container Terminus commits the U.S.-flag carrier to use Rotterdam as its north European hub until the year 2013.Robert Hintz, Sea-Land's chief executive officer, will attend an official ceremony at which ECT President Gerrit Wormmeester will unveil a plan to build a $300 million container terminal to serve Sea-Land.

ECT and Sea-Land signed a letter of intent in December ending year-long speculation that the U.S. carrier might pull out of Rotterdam after the expiration of its 20-year contract in 1990 and move to either Antwerp or Zeebrugge.

Loss of the Sea-Land contract, totaling about 400,000 containers a year, would have threatened the survival of ECT and undermined Rotterdam's position as Europe's premier containerport.

ECT has recouped most of the 200,000 containers a year lost in the late 1986 bankruptcy of United States Lines and capacity utilization at its new Delta terminal has jumped from a low of 25 percent to 65 percent.

Sea-Land will stay at ECT's Home Terminal until the end of 1992 before moving to the new terminal at the Maasvlakte, Rotterdam's outer port, in January 1993.