The Office for National Statistics (ONS) has not been including night arrivals in its estimates of immigration to the UK, meaning that it could be far higher than officially claimed.

British net immigration statistics are not based on a firm count of arrivals versus departures. Instead, the ONS makes an estimate based on the International Passenger Survey (IPS), a tool devised in the 1960s in order to provide information about the tourism industry.

One of its deficiencies, reports The Sun newspaper, is that survey teams only operate between 6 a.m. and 10 p.m. Sixteen flights arrive every day between 4:30 a.m. and 6 a.m. at Heathrow alone, largely from Asia – and the ONS will now sample these to see if it is missing a large number of, in particular, foreign students.

The decision comes as new figures reveal some 50,000 foreign students are staying on in the UK after completing their courses, despite a crackdown on bogus colleges which was supposed to stop migrants abusing the higher education system as a route to permanent migration.

The International Passenger Survey interviews around 300,000 people per year – just 0.2 per cent of the 200 million people who enter and leave the United Kingdom annually. The country’s smaller air and sea ports are not surveyed at all, and nor is the land border with the Republic or Ireland.

Travellers are under no obligation to agree to be interviewed, and there is no way to guarantee they will answer questions honestly. Naturally, it is difficult for IPS teams to glean any useful information from arrivals with poor or non-existent English language skills.

Commenting on the revelations, Migration Watch vice-chairman Alp Mehmet told The Sun that the ONS “really does have a job on its hands to convince us that their figures are indeed as accurate as they claim”.

This is not the first time the IPS has come in for criticism: in November 2006, then-Governor of the Bank of England branded the survey “hopelessly inadequate”.

King told the Treasury Select Committee that, when the EU made Britain open its borders to Central Europe in 2005, the number of passenger journeys between Poland and the UK surged from roughly 516,000 to 1.8 million – yet just 79 people surveyed outside Gatwick, Heathrow, and Machester airports told the IPS they were migrants.

The ruling Conservative Party was also extremely critical of the IPS while in opposition, with an inquiry chaired by Michael Fallon – now Secretary of State for Defence – branding it “not fit for purpose, having been designed to provide data for tourism and business travel purposes”