By Nick Childs BBC Washington Correspondent

The new measure comes into force on Wednesday.

It may be one of the most significant new layers of security introduced so far at American airports, and another measure of how things have changed since the 11 September attacks.

Before then, only 5% of the luggage at American airports was screened.

Airlines are threatened by bomb attacks

Now, by order of Congress, every one of the more than one billion items checked in by passengers each year must be scanned for explosives.

The cost in new scanning equipment is estimated at over $1.3bn.

Not quite all of America's 429 main commercial airports yet have the equipment.

But the authorities say sniffer dogs, hand searches and checks against passenger lists will fill in the gaps for now.

Some critics say the stop-gap measures are not enough.

There are also concerns that introducing the new screening over the New Year holiday will bring travel chaos.

Again, the authorities believe they have taken precautions.

The new measures will mean longer queues and more delays, but all the signs are that America's travelling public is ready to pay the price.