British border guards have been banned from X-raying lorries while searching for illegal immigrants at French ports in case the radiation harms the stowaways' health, it was revealed today.

Lucy Moreton from the Immigration Services Union said that her members are only permitted to use the scanners at ports in France when searching for smuggled or illegal goods.

The revelation comes as it emerged that at least 24 of the migrants who made it to Britain hiding in the back of two lorries earlier this week have already been given their freedom.

Security: A file image of a lorry X-ray machine searching a vehicle at Southampton Docks. British border guards have been banned from X-raying lorries while searching for illegal immigrants at French ports

X-ray: The Immigration Services Union's general secretary said that her members are only permitted to use the scanners at ports in France when searching for smuggled or illegal goods

Mrs Moreton told The Times: ‘The French will not allow us to use them for looking for illegals. They only allow us to use scanners to search for contraband, not people.’

Speaking to the newspaper’s reporters Richard Ford and Sean O’Neill, the ISU general secretary added that the machines are 'very, very slow to operate, taking about an hour per each vehicle'.

Also yesterday, Mrs Moreton suggested that increased borders checks in the wake of the Brussels attacks are likely to only last for two weeks because the ‘stepped up’ level cannot be maintained.

Mrs Moreton - from the ISU, which represents border agency and immigration staff - said checks had been ‘raised’ at border points but would significantly disrupt freight if they continued long-term.

She told BBC Radio 4's Today programme: ‘There's quite a lot of immediately available money for very high profile types of interventions.

Union spokesman: Lucy Moreton (pictured in a previous BBC interview) said the French 'will not allow us to use them for looking for illegals - they only allow us to use scanners to search for contraband, not people'

Found: 20 Iranians and five Iraqis were discovered packed inside the back of the lorry pulled over in Kent

‘We've stepped up controls at the border, and at all borders, but it is maintaining that at a high level for a long period of time and whether there is the political will, or potentially even the necessity to do so.’

The French will not allow us to use them for looking for illegals. They only allow us to use scanners to search for contraband, not people Lucy Moreton, Immigration Services Union

Asked if there was the political will, she replied: ‘Experience from past would suggest that it isn't. The increased checks at the border last about two weeks, or that's how long they lasted after the Paris attacks.’

Meanwhile, an independent report from the Independent Chief Inspector of Borders and Immigration found plans to remove foreign criminals and illegal immigrants were cancelled in 40 per cent of cases.

And last night it was revealed at least 24 of the migrants who made it to the UK hiding in the back of two lorries in recent days have already been set free.

All 51 stowaways had claimed asylum and the remaining 27 men, women and children who were discovered sneaking into the country were expected to have been let out by last night.

On the roadside: 26 migrants were found in the back of a fruit and vegetable lorry near Canterbury in Kent

Victory signs: Two of the illegal immigrants found in Canterbury make the gesture. They were among 24 men, one woman and boy found in the back of a Romanian-registered HGV while others were found in another lorry

One of the trucks arrived in the UK on Wednesday, hours after Home Secretary Theresa May announced toughened border security following the Brussels suicide bombings in which 31 were killed and 270 wounded.

We've stepped up controls at the border, and at all borders, but it is maintaining that at a high level for a long period of time and whether there is the political will, or potentially even the necessity to do so Mrs Moreton

Photographs of stowaways crammed like sardines in the back of one lorry were published earlier in the week.

It was a damning illustration of how Britain has lost control of its borders – and fuelled fears about the rigour of security checks during the current terror alert.

All 51 migrants are understood to have claimed they were fleeing war, persecution and humanitarian disaster in Iran, Iraq and Syria.

By yesterday most had been processed at the Kent Intake Unit, where they were offered food, drink and medical attention, and underwent security screening.

After this was done, they were officially classed as asylum seekers – then released from the centre to specialist accommodation or to live with family with instructions only to report in intermittently.