CONCORD, N.H. – Nearly a decade after it was first brought, a lawsuit accusing two oil giants of widespread groundwater contamination in New Hampshire is expected to present jurors with the most complex and time-consuming trial in state history.

The products liability case against ExxonMobil and Citgo will be tried beginning in mid-January in a federal courtroom — on loan to the state — because it would undermine the rights of criminal defendants to a speedy trial if it tied up one of the three courtrooms in Merrimack Superior Court, officials said.

The state sued 26 oil companies and subsidiaries in 2003, claiming the gasoline additive MTBE, methyl tertiary butyl ether, caused groundwater contamination in a state where 60 percent of the population relies on private wells for drinking water.

New Hampshire is seeking more than $700 million in damages to test and monitor every private well and public drinking water system in the state and to cover cleanup costs where needed.

New Hampshire is the only state to have reached the trial stage in a lawsuit over MTBE. Other lawsuits have been brought by municipalities, water districts or individual well owners, and most filed in the past decade have ended in settlements. New York City in 2009 won a $105 million federal jury verdict against ExxonMobil for MTBE contamination of city wells; that verdict has been appealed.

MTBE had been used in gasoline since the 1970s to increase octane and reduce smog-causing emissions. While it was credited with cutting air pollution, it was found in the late 1990s to contaminate drinking water when gasoline is spilled or leaks into surface or groundwater.

The U.S. Environmental Protection Agency had classified it as a “possible human carcinogen.” New Hampshire banned its use in 2007.

All the sued oil companies but ExxonMobil, based in Irving, Texas, and Citgo, based in Houston and owned by Venezuela, have reached settlement agreements with the state. Just last month, Shell Oil Co. and Sunoco Inc. agreed to pay the state a total of $35 million.

When the lawsuit was originally filed, then-Attorney General Peter Heed said MTBE contamination had caused an “unprecedented environmental problem.” Heed said MTBE “has been associated with adverse health consequences and can render water unpalatable.”

The case was tied up in federal courts for years on jurisdiction issues before being sent back to state court.

Lawyers for the defendants claim that they are not liable and that MTBE functioned as it should. They also stress that they have cleaned up their own sites and that contamination elsewhere was caused by third parties who have not been sued.