Tens of thousands of illegal migrants are being discovered at U.K. borders, leading campaigners to believe that record numbers have made their way into the country.

Sir Charles Montgomery, the director general of the Border Force, said 30,000 would-be stow-a-ways were found by British officers at Calais, where the border has been moved to, with a further 6,000 detained on U.K. soil, the Daily Mail reports.

This is double the 18,000 caught between 2013 and 2014, he said.

But with the huge numbers trying to get into this country, lured by social security benefits and council housing as well as the NHS and free schooling, the numbers are likely to be in the hundreds of thousands.

Sir Charles told the Home Affairs Select Committee it was not possible to stop all illegal immigration. And he admitted not a single person arriving by train or coach, estimated at some ten million, is counted in or out of the country, or checked against criminal watch lists.

He told MPs, “‘I cannot look this committee in the eye and say I can operate 100 per cent control so 100 per cent of clandestines will not get across the border.”

Exit checks are due to start on April 8, with holiday makers warned they could face huge queues at Eurotunnel and ferry ports. The start date has already been delayed by a week to avoid launching the initiative over the busy Easter weekend.

Officials will now have to record the passport details of all passengers and pass details on to the home office. Coach parties of under 16-year-olds would be excluded to reduce delays, Sir Charles said, leaving open an opportunity for illegal trafficking of children.

“The coach sector is the most difficult of all logistically and that has been the focus of trials and testing,” he said.  “We still have to refine the process to ensure it works as smoothly as possible. I’m confident we will have a solution come the live date.”

As well as £12 million which the British gave to Calais to assist with security at the main entry point to the country, border staff use dogs, carbon dioxide detectors, heartbeat monitors and scanners.

Despite this, transport firms have said that rather than deport any captured stowaways back to their country of origin, France just lets them free — to let them try to re-enter Britain.

Calais Mayor Natacha Bouchart has called on the government to do more, especially to dispel the myth that Britain is an “El Dorado.” She has even threatened to undo the Treaty which moved the border from Dover to Calais to allow the Border Agency to set up in the French port city.

She is aware that the migrants, who set up unsightly, slum-like camps in the City, have no wish to stay in Calais, but are just waiting there with a do-or-die attitude for the next opportunity to board a vessel which will get them to Dover.

Most of the migrants have claimed asylum when they arrived in a Southern European country such as the Italian town of Lampedusa, Greece or Malta. Once they are approved by one EU member country for asylum, under the Dublin Treaty, no other country has the right to say they are not refugees or asylum seekers.

The Government has also negotiated access to the Second Generation Schengen Information System, known as SIS II, which has details of a quarter of a million wanted or missing people.

UKIP MEP for Dover Janice Atkinson said the delays being imposed on British passport holders was the Home Secretary trying to “sound tough on immigration.” Instead, she said, she “just sounds UKIP-lite.”

“Here’s a novel idea: why not let the British passport holders through without added checks, whilst those with non-British passports go through more rigorous checks? As much as the majority of British citizens would think this a very sensible idea we cannot do this as it is illegal under EU law. Therefore, families, holidaymakers and business people will face unnecessary checks as once again, a one-size-all-policy from this government or the EU doesn’t work.”