The number of foreigners who overstay their visas has nearly doubled to more than 90,000 a year amid fears that lax immigration checks are allowing people to remain in the UK illegally.
It comes on top of more than 500,000 non-visa national visitors who were not recorded as leaving on time in a two-year period analysed by the chief inspector of borders.
New analysis by the Migration Watch think tank reveals that 4.8 per cent of the 1.9 million people who came to the UK on work or study visas did not leave on time – equivalent to 91,900 people and up from 49,400, or 3.7 per cent, in 2016-17.
However, these figures do not cover the largest tranche of non-EU visitors – so-called "non-visa nationals" who are citizens of 55 countries who do not need a visitor visa in order to come to the UK for a six-month stay. They include millions of visits per year from the United States, Mexico, Brazil and, in the wake of Brexit, also include EU visitors.
The most recent investigation by the Independent Chief Inspector of Borders and Immigration (ICIBI) found that the number of non-visa national visitors not recorded as leaving on time between 2015 and 2017 amounted to 500,000 – just over 250,000 per year.
The Government has declined to release recent figures despite repeated parliamentary questions earlier in the year.
Alp Mehmet, the chairman of Migration Watch UK, said: "Overstaying is very likely getting worse, while the Government seems to have ignored a large portion of data which could clarify the picture. So much for promises to take back border control.
"Our research points to a major problem, and it is about time the Government told us how they are going to tackle it."
The Government introduced a new points-based post-Brexit immigration system last month, designed to end the UK's dependence on "cheap labour" from Europe. While limiting visas to low-skilled workers, it makes it easier for higher-skilled workers to get them.