Classified log of individuals banned from flying into or within America as they are considered a threat stands at 21,000

The size of the US government's secret list of suspected terrorists who are banned from flying to or within the country has more than doubled in the past year.

The no-fly list jumped from about 10,000 known or suspected terrorists one year ago to about 21,000, according to government figures. About 500 are US nationals.

The flood of new names began after the failed Christmas 2009 bombing of a Detroit-bound jetliner when the US government lowered the standard for putting people on the list and scoured its files for anyone who qualified. "We learned a lot about the watchlisting process and made strong improvements, which continue to this day," said Timothy Healy, director of the Terrorist Screening Centre, which produces the no-fly list.

Among the most significant new standard is that a person doesn't have to be considered only a threat to aviation to be placed on the list.

People considered a broader threat to domestic or international security or who attended a terror training camp are also included, said a US counter-terrorism official who spoke on condition of anonymity. As agencies complete the reviews of their files, the pace of growth is expected to slow, the counter-terrorism official said.

The American Civil Liberties Union has previously sued the US government on behalf of Americans who believe they are on the no-fly list and have not been able to travel by air for work or to see family.

"The news that the list is growing tells us that more people's rights are being violated," said Nusrat Choudhury, of the ACLU. "It's a secret list, and the government puts people on it without any explanation. Citizens have been stranded abroad."

People who complain they're unfairly on the list can submit a letter to the homeland security department, but the only way they'll know if they're still on the list is to try to fly again, she said.

While the list is secret, it is subject to continuous review to ensure the right people are on it and that those who shouldn't be are removed, said Martin Reardon, a former chief of the FBI's terrorist screening operations centre.

If a person is nominated to be on the no-fly list, but there is insufficient information to justify it, the person is downgraded to a different list, he said. "You can't just say: 'Here's a name. Put him on the list.' You've got to have articulable facts."

On average, there are 1,000 changes to US watchlists each day, most of which involve adding new information about someone on the list.

The no-fly list previously swelled to 20,000 people in 2004. At the time, people including the late Ted Kennedy, a sitting senator, were being stopped before flying.

The US transportation security administrator, John Pistole, said instances of travellers being mistaken for terrorists were, however, down significantly since the US government and not airlines became responsible for checking the list.

Travellers must provide their full name, birthdate and gender when purchasing an airline ticket so the government can screen them against the terror watchlist.