A terror suspect and sex offender with 17 aliases has reportedly been working at Britain's busiest airport in an alarming security breach.

Career criminal Mohamed Abdullahi Mohamud is said to be on an official terror watchlist but reportedly had access to Heathrow's runway.

Mohamud, 31, has a criminal record stretching back 11 years including robbery, assault, money laundering and threats to kill.

Mohamed Abdullahi Mohamud, a terror suspect and sex offender with 17 aliases, has reportedly been working at Britain's busiest airport in an alarming security breach (file photo)

But airport bosses allegedly failed to carry out a full background check and he was handed a security pass and started work as a kitchen porter in a first-class lounge, according to the Sunday People.

A security source told the paper: 'How can somebody with a criminal past like this slip through the net?

'Even if he hadn't been a terror suspect his convictions should have meant he wasn't allowed anywhere near a plane – let alone our busiest airport.'

Mohamud is said to be known to the security services and has been interviewed five times in the past two years by anti-terror officers, it is claimed.

He reportedly had two terror charges - relating to alleged hoaxes involving a harmful substance and a bomb - dropped due to insufficient evidence.

He was convicted of sexual assault in 2015 and ordered to sign the sex offenders register for 10 years and in total has served five years in prison.

Mohamud has been interviewed by Prevent, the Government's anti-radicalisation programme, but he contacted a newspaper this week telling them he had been offered a job at an airport.

He boasted no background checks were made and claimed he wanted to expose a security loophole.

Mohamud, 31, has a criminal record stretching back 11 years including robbery, assault, money laundering and threats to kill (file photo of Heathrow airport)

The sex offender even took a picture of a plane from the runway to prove how close he got to the aircraft.

It emerged he had lied about his lengthy criminal record to recruiters and took up the job through the SM Global Consultancy Ltd agency on Monday.

On Tuesday he arrived at Heathrow and began work at the business and first-class lounge.

Frighteningly, he spent five days at the airport and had airside access in Terminal 3.

Around 17.7 million passengers pass through the terminal on 89,000 flights each year.

No full background check was carried out before he arrived at the airport and he could have gone undetected for up to a month because a full criminal record check through the Disclosure and Barring Service search could have taken four weeks.

Airport bosses allegedly failed to carry out a full background check and he was handed a security pass and started work as a kitchen porter in a first-class lounge (file photo)

His security pass was revoked when airport bosses were alerted to the breach and Mohamed was sacked.

A spokesman for SM Global Consultancy told the People all security procedures set by Heathrow Airport and the Civil Aviation Agency had been followed.

A Heathrow spokesman said: 'We would like to reassure those who may be worried about this story that no security breaches have been made and Heathrow airport remains safe.

'The conditions of this temporary pass required the individual to be constantly escorted by a permanent member of staff. He was also subjected to robust and thorough airport security search, each time he gained access to certain areas.

'The UK has some of the most robust aviation security measures in the world and Heathrow follows and is compliant with Government regulations, which ensured checks were in place to limit airport access to this individual.'