The US military is allowing convicted housebreakers, muggers and drunk drivers who have killed pedestrians to join up to fill the growing gap between necessary frontline fighting strength and recruitment.

Thousands of volunteers for the Army and Marines have been granted "moral waivers" in the past three years to allow them to become soldiers despite having criminal felony records for offences from robbery to drug-taking.

The only candidates still barred from a military career are convicted drug dealers, sexual offenders and murderers.

The number of felony waivers granted by the Army for serious crimes jumped from 411 in 2003 to 901 last year while waivers for misdemeanours such as petty theft and minor drug possession rose from 2700 to more than 6000.

Pentagon officials insisted yesterday that the waiver programme was "a way to admit young people who had made a mistake, but had overcome past bad behaviour".

Paul Boyce, Army spokesman, added: "Anything that is considered a risk or a serious infraction of the law is given the highest level of review.

"Our goal is to recruit quality young men and women who can contribute to defending America against its enemies."

Congressman Marty Meehan, a Massachusetts Democrat, said he and fellow legislators were "concerned" the struggle to fill the ranks had caused standards to fall.

"By pursuing this course, we are potentially endangering the rest of our armed forces and sending the wrong message to recruits across the country," he said.

The US military has also raised the upper age for enlistment from 35 to 42 and is accepting candidates who fail aptitude tests.