Holidaymakers have been urged to get to airports early as it emerged new security checks prompted by fears of a terrorist plot could last indefinitely.

The exact nature of the new security measures have not been disclosed, but it is thought they include extra searches of passengers just before they board a flight, with extra scrutiny of shoes and electronic devices being carried on to planes.

Travellers may be called to gates earlier to go through the additional layer of checks, particularly for America-bound flights.

The new measures follow what are understood to have been reports from US authorities that terrorists are working on a bomb that could sidestep current checks, including body scanners, to bring down a plane.

The threat is thought to have emerged after terror groups in Yemen and Syria joined forces to plan an attack.

But there are also fears individuals with Western passports who have travelled to Syria and Iraq in recent months to fight with Islamist extremists could be used to smuggle devices on to aircraft.

Prime Minister David Cameron said no risks would be taken with the safety of passengers which "must come first". Downing Street also said travellers should make sure to give themselves plenty of time in the airport.

Asked if the public should be understanding about the potential for delays, the Prime Minister's official spokesman said passengers "will understand why it is necessary to have security measures in place".

He added: "The advice from all of Government would be for people to continue to follow the advice from airports and airlines.

"That includes, as it has done for a very long time, giving themselves the appropriate time to go through all the airport procedures."

Earlier, the Department for Transport said the majority of passengers "should not" experience significant disruption to their journey through airports.

But Deputy Prime Minister Nick Clegg warned the new checks were unlikely to be "a one-off, temporary thing". He added: "This is the world we now live in," as he suggested terror groups were determined to get past current security measures.

He added: "This is part of an evolving and constant review about whether the checks in our airports - and indeed other places of entry and exits from countries - keep up with what we know from intelligence and other sources about the nature of the threats we face."

Intelligence is thought to have suggested bomb-makers linked to the terrorist group al Qaida in the Arabian peninsula (AQAP), which is based in Yemen, have travelled to Syria to meet the al Qaida-affiliated Jabhat al-Nusra.

It is thought the two groups and working together to develop an explosive device that will be undetected by existing airport security.

Al Qaida operatives in Yemen were behind an underwear bomb that failed to detonate aboard a jet over Detroit on Christmas Day 2009.

A more sophisticated version of the same explosive device was uncovered during an operation by the CIA in 2012.

Saudi-born bomb-maker Ibrahim Hassan al-Asiri, believed to be a member of AQAP, was also said to be behind a powerful bomb hidden in printer ink cartridges that was intercepted at a UK airport en route to America in 2010.

The device had been timed to go off as the plane it was destined for was travelling over the US.

Downing Street described the latest measures as a response to what they said was an "evolving threat" from terror groups.

The Prime Minister's official spokesman also said the new checks only affected airports, not train stations or ports.