RICHMOND, Va. — The U.S. Department of Transportation says the use of smokeless electronic cigarettes on airplanes is prohibited and plans to issue an official ban this spring, according to a letter from Transportation Secretary Ray LaHood obtained by The Associated Press.

In the letter to Sen. Frank Lautenberg of New Jersey, LaHood said the department has been informing airlines and the public that it interprets smoking regulations to include e-cigarettes. Lautenberg, who wrote the 1987 law that banned smoking on airplanes, had asked transportation officials to clarify the rule.

E-cigarettes are plastic and metal devices that heat a liquid nicotine solution in a disposable cartridge, creating vapor that the "smoker" inhales. A tiny light on the tip even glows like a real cigarette. They have prompted debate over how risky they are and whether they’re even legal.

Numerous videos on YouTube show passengers using the devices on airplanes. Lautenberg even said some passengers have interpreted flight attendant instructions to mean that the devices were only prohibited when other electronic devices were not allowed during takeoff and landing.

Many airlines already have begun informing passengers that the devices are not allowed on flights, but Lautenberg said there had been confusion over their use and wanted to make sure officials were solidly opposed to opening the door to e-smoking on planes. Some e-cigarette distributors have touted their convenience because they can be "smoked" anywhere traditional cigarettes are not allowed.

"We still don’t know the health effects of e-cigarettes, and we don’t want to turn airline passengers into laboratory mice," Lautenberg said in an e-mailed statement.

Jason Healy, president of e-cigarette maker Blu Cigs, called the move somewhat disappointing, but said it isn’t the end of the world. Healy said he hopes that once more people understand e-cigarettes and more testing is done, that airlines could choose whether to allow them.

"I understand from an airline’s point of view the hassles it could create," said Healy, whose company has partnered with a luxury charter jet company to provide free e-cigs on private flights. "It’s not the actual product, it’s the disruption and explaining to everyone else that it’s not smoke."

Users and distributors say e-cigarettes address both the nicotine addiction and the behavioral aspects of smoking — the holding of the cigarette, the puffing, seeing the smoke come out and the hand motion — without the more than 4,000 chemicals found in cigarettes.

Nearly 46 million Americans smoke cigarettes. About 40 percent try to quit each year, according to the Centers for Disease Control and Prevention. But unlike nicotine patches or gums, e-smokes have operated in a legal gray area.

First marketed overseas in 2002, e-cigarettes didn’t become easily available in the U.S. until late 2006. Now, the industry has grown from the thousands of users in 2006 to several million worldwide, with tens of thousands new e-smokers every week.

The future of e-cigarettes is likely to be decided by the Food and Drug Administration. The FDA lost a court case last year after trying to treat e-cigarettes as drug-delivery devices, rather than tobacco products, because e-cigarettes heat nicotine extracted from tobacco.