Ibrahim

Experts believe al-Qaida bomb-maker could be working with ISIS - and that jihadists with western passports might target planes

Concern about the prowess of al-Qaida's bomb-maker in chief - and his willingness to work with Isis insurgents in Syria and Iraq - underlies the decision to increase security at British and other European airports.

Ibrahim Hassan al-Asiri, a Saudi chemist who became a bomb-maker, has for years been high on America's most-wanted list, because he is believed to be behind many audacious attempts to bring down transatlantic flights, using his skills as a chemist to devise increasingly imaginative ways to conceal explosives, with the best known being the "underwear" bomber.

The new element that led to the present scare is intelligence linking Asiri for the first time to two groups in Syria and Iraq, the Nusra Front and the Islamic State of Iraq and the Levant (Isis).

The worry is that Asiri's skilfully disguised bombs might be carried on to transatlantic airlines by passport-holders from the US or Europe.

Asiri has survived several assassination attempts in Yemen, the latest a drone attack in April. He has been reported killed, only to resurface.

The other reason for the heightened threat is the increasingly familiar warning from US, British and other European intelligence agencies who have been voicing concern for more than a year of the threat posed by the thousands of young jihadists from America and Europe who have joined the fight in Syria and now Iraq.

The heightened security in Europe is primarily for transatlantic flights and comes at a peak travel period for the US, the 4 July Independence Day holiday.

Politicians said the changes would be permanent, with David Cameron saying the safety of passengers "must come first". The prime minister said he hoped the measures would not cause unnecessary delays, but stressed that no risks could be taken with passenger safety.

He told the BBC: "We take these decisions looking at the evidence in front of us and working with our partners.

"This is something we've discussed with the Americans, and what we have done is put in place some extra precautions and extra checks."

The government highlighted the importance of vigilance, but said the extra security measures - details of which have not been disclosed - were not expected to cause "significant disruption" to passengers. The official UK threat status remained unaltered at "substantial", the third grade in the five-level rating.

Earlier on Thursday, the deputy prime minister, Nick Clegg, warned of the dangers posed by a "medieval, violent, revolting ideology".

He said that "I don't think we should expect this to be a one-off temporary thing" that was part of what he described as "an evolving and constant review about whether the checks keep up with the nature of the threats we face".

The secretary of the US department of homeland security, Jeh Johnson, said on Wednesday that information about the aviation industry was being shared with "our foreign allies", while US officials told Reuters the increased security at European airports was because al-Qaida operatives in Syria and Yemen had teamed up to develop bombs that could be smuggled on to planes.

Jonathan Wood, a global issues analyst at security consultants Control Risks, judged the present threat "plausible" given that the leader of al-Qaida in Yemen called for an attack on the US in a video in April.

Wood said he did not know of anyone who had been personally trained by Asiri going to Syria or Iraq, but it was not improbable that some of his experience had ended up in these countries, given the link between al-Qaida and Nusra.

Asiri, 32, was born in Riyadh, grew up near the Saudi border with Yemen, studied chemistry at university in Riyadh and fought in Iraq.

There is no suggestion that Asiri is in Syria or Iraq training militants or that he has sent them any devices. It is thought to be more likely that people trained by him or devices he has designed have ended up in Syria or Iraq.

US intelligence officials, briefing reporters, warned of creatively designed, non-metallic explosive devices that could avoid detection. But they do not have a specific device or design in mind.

Asiri has proved inventive, even if so far largely unsuccessful. In the December 2009 attempt to detonate a bomb on a flight from Europe to Detroit, the explosive was hidden in the underpants of a Nigerian.

In a separate attempt, British bomb disposal experts found explosives hidden in printer cartridges in a courier package in 2010.

Some of the designs attributed to him sound like the stuff of fantasy, such as having explosives surgically implanted. But he may already have tried this.

One of the most audacious, imaginative and ruthless attacks was a suicide attack in Saudi Arabia in 2009.

The attacker was his younger brother and the target the deputy Saudi interior minister, and his brother was killed. The bomb was either hidden in his rectum - or surgically implanted.