Around 300 potential terrorists, criminals and illegal immigrants try to enter Britain each day, with 99,500 alerts issued by the National Border Targeting Centre (NBTC) last year.

Figures show the number of potentially dangerous passengers being detected has risen by 60 per cent over the last couple of years, including 46 people travelling to Syria over the past six months alone.

The Mail on Sunday says that the Home Office has not said how many of these alerts led to people actually being blocked from entering the UK, but different figures show some 18,000 people were prevented from entering the country last year. Nearly 5,000 of them were at the French port of Calais.

The huge rise in the number of alerts has coincided with the rise of Islamic State and the continued terror threat to the UK.

The NBTC, which was once dubbed “Britain’s Stasi” because it screens all passengers entering or leaving Britain, receives its data from airlines. It is informed of passengers’ names, dates of birth, nationalities and passport numbers.

It can even obtain people’s travel history dating back over decade, meaning even those with no criminal history can be flagged as potential threats.

The organisation was set up in 2010, and in its first year sent out 61,000 alerts to border staff or local police. In the 2012/13 year – the year of the London Olympics – that figure rose to 62,000.

However, in 2013/14 the number suddenly rocketed to 91,000 alerts, and in 2014/15 rose even further to 99,500 – equivalent to 273 a day.

Lord Carlile of Berriew, a terror analyst, said the increased number of alerts was due to an ever greater threat from jihadists:

“Given what has happened in the past year and the fragility of the European border system, the authorities would be acting irresponsibly if there were not more alerts.

“The border agencies are doing the best they can in a situation in which the threat level is severe and they do act on an intelligence-led basis. That means they sometimes get it wrong, but I would rather they get it wrong on a small percentage of cases than there was a bomb in Oxford Street.”

The figures come just weeks after it was revealed UK authorities are allowing potential terrorists to leave the UK freely if they believe it would be more dangerous to keep them on British soil.

One counter-terror officer said the strategy was called the “home and away debate”:

Do you let them go? Are they going to be dangerous overseas or more dangerous here? It is a dilemma for us and it is not an easy one.

“It depends on other factors at play. It depends on where they are going to go and what they are going to do.

“If the risk of them carrying on being radicalised and carrying on this activity is reduced by them going to another country then there may be a consideration to let them go because they are more of a risk here and more of a threat here with the company they are keeping.”