Updated

Homeland Security Secretary Michael Chertoff rebuked lawmakers Friday for seeking to stall new rules on driver's licenses that could cause big headaches for air travelers starting in May.

Federal authorities are currently at a standoff with a handful of states over a law called Real ID, which would require new security measures for state-issued driver's licenses.

South Carolina, Maine, and Montana are the only states that have not sought extensions to comply, or already started toward compliance with Real ID, which was passed after the 2001 terror attacks on New York and Washington.

A fourth state, New Hampshire, has asked to be exempted, but homeland security officials do not view that letter as a legally acceptable request, so the Granite State has not received an extension.

Chertoff has warned that if holdout states do not send a letter by the end of March seeking an extension, come May, residents of such states will no longer be able to use their driver's licenses as valid ID to board airplanes or enter federal buildings.

Such travelers would instead have to present a passport or be subjected to secondary screening.

Five senators — Susan Collins and Olympia Snowe of Maine, Jon Tester and Max Baucus of Montana, and John Sununu of New Hampshire — appealed to Chertoff last week to exempt all 50 states from the looming deadline.

Chertoff responded Friday that it was not he, but Congress who picked the date when the law went into effect in 2005.

"You may disagree with the foregoing law, but I cannot ignore it," Chertoff said in the letter.

The law, he said, is necessary for national security according to recommendations from the commission that studied the Sept. 11, 2001 attacks.

"Secure identification is a cornerstone of protecting our communities," he said.

The nation's top homeland security official also offered a blunt warning to those critics who claim the government is bluffing when it says it will impose harsher security reviews in states that do not seek an extension from the Real ID law.

"Showing up at the airport with only a driver's license from such a state will be no better than showing up without identification," he wrote. "No doubt this will impel many to choose the inconvenience of traveling with a passport."

Chertoff has offered a plan to gradually implement Real ID requirements over a period of ten years, so that eventually all driver's licenses would have several layers of security features to prevent forgery. They would also be issued only after a number of identity checks, including immigration status and verification of birth certificates.

Critics of the plan say it is too expensive, an invasion of privacy, and won't actually make the country safer.

The most outspoken, Montana Democratic Gov. Brian Schweitzer, has said the federal government can "go to hell." He argues that Real ID won't work and the Bush administration won't be around long enough to prove it.