Border Force check the passports of passengers arriving at Gatwick Airport Britain’s borders were left exposed to terrorists last year after a Home Office computer system which screens passengers crashed twice in 48 hours, The Telegraph can reveal.
The eBorders system, which was put in place after the 9/11 terror attacks to protect the country from jihadists, ground to a halt in June last year.
The Home Office refused to reveal how often the system has crashed or whether there have been any outages since the incident.
Technicians worked through the night to fix the system amid fears from border officials that hundreds of extremists, convicts and illegal immigrants were arriving in the UK undetected.
The disclosure that a vital part of Britain’s border security stopped working during a time of “severe” threat from terrorism will raise serious questions about whether it is fit for purpose.
Mrs May is likely to come under pressure to explain why the public were kept in the dark despite tens of thousands of people likely to be traveling into the UK at the time.
Flights were not grounded despite the system being down and border officials unable to check in advance passenger details against terrorism watch lists.
Sources familiar with the situation told the Daily Telegraph that they would normally expect to see the names of hundreds of potential suspect passengers being ‘flagged’ each day.
During the outages just one individual was ‘flagged’ suggesting others could have gone under the radar.
A Home Office spokesman said that all passengers would still have had to cross passport control after arriving in the UK.
Officials at the border have access to lists of “dangerous” people, which the Home Office insists would have been cross-checked to ensure there was no breach.
However the warnings index – which dates back to 1995 – was deemed inadequate on its own after the 9/11 attacks and was recently found to be breaking down twice a week .
But terrorists or criminals could still have boarded aircraft without being detected by British security services.
The Telegraph has launched a new Border Security campaign, which has seen former counter terrorism figures call for a review in the wake of terror attacks on the Continent.
At the heart of Britain’s ability to stop dangerous people entering the country is Semaphore, a system which checks passenger data against watch lists of suspect individuals.
Every day Semaphore scans information on passengers traveling to and from Britain on planes, trains and ferries against lists of those flagged up by government agencies.
The system – unlike its predecessor – helps alert the border agencies to suspect passengers bound for the UK before they board planes.
Matches are passed to the National Border Targeting Centre (NBTC) which decides whether to stop the passenger from boarding, intercept them at passport control or let them enter the UK.
This newspaper has learnt that on Sunday, June 14, and Monday, June 15, the Sempahore system suffered two national outages after being overwhelmed by requests.
The first crash happened when a fault saw tens of thousands or error messages flood the system which froze under the pressure.
The system appeared to stabilise before another malfunction saw hundreds of thousands of passenger details flood the system and trigger another outage on Monday night. Mrs May was notified again.
Officers at the NBTC warned that instead of seeing hundreds of matches they had received just one – meaning potential criminals and jihadists heading to Britain were not being flagged up.
Specialists worked through the night again trying to locate the source of the problem before finally stabilising the system on Wednesday.
They occurred just months after the Charlie Hebdo shooting that saw jihadists kill 11 people in Paris and while Britain’s threat level was set at “severe”, meaning a terrorist attack is “highly likely”.
It raises questions about whether Semaphore, which was meant to have been scrapped five years ago and will remain in place until at least March 2019, can be relied upon.
The failures were also not cited in inquiries into Britain’s eBorders system by the National Audit Office and the Commons Public Accounts Committee, who both published reports after the incident.
A Home Office spokesperson said: “Protecting our border is, and always has been, of paramount importance to the Government.
“Throughout these incidents, we maintained full checks on 100 per cent of arriving passengers using the Warnings Index – which is a completely separate to Semaphore, our pre-departure advance checking system. This ensured no passenger who was the subject of a Warnings Index entry entered Britain.
“We use a number of advanced technology systems as part of our multilayered approach to border security. As well as the Warnings Index and Semaphore, we run checks on the Schengen Information System – which provides law enforcement with real-time alerts on wanted criminals and terrorists – and collect data on exit checks.
“These systems, in conjunction with a range of programmes across security and law enforcement, ensure we keep our citizens safe and our country secure at all times.”
A Home Office source added that “all live IT systems are susceptible to outages”.
A BA spokesman said: “We work closely with the Home Office to ensure the smooth and timely transfer of data.”
Jack Dromey, the Labour shadow police minister, said: “Strong borders, well policed, are crucial for the safety and security of our country.
“At a time when our country is facing a uniquely awful threat of terrorism, we must be able to keep out those who would bomb, kill and maim.
“If our eBorders system has twice crashed then the Home Office should have come clean with the public. Theresa May and the Home Office have very serious questions to answer about what went wrong.”