Are there 1.2 million ‘illegal immigrants’ in the UK?

We’ve seen a number of Reform UK politicians claim that there are 1.2 million “illegal immigrants” in the UK.

The figure has been referenced by the party’s leader Nigel Farage MP, former party chair and current head of Reform UK’s ‘Department of Government Efficiency’ Zia Yusuf and Greater Lincolnshire mayor Andrea Jenkyns.

And last month, a claim that the UK could save £10 billion a year by “deporting the 1.2 million illegal migrants currently in Britain” circulated widely on Facebook.

The 1.2 million figure appears to be based on research published by the Pew Research Center think tank in 2019, which estimated that in 2017 between 800,000 to 1.2 million unauthorised immigrants lived in the UK. But there are a number of issues with this figure, and the Pew Research Center itself acknowledged earlier this year that its estimate was flawed.

Statistics on their own have limitations. The way they are presented is a crucial part of how they are interpreted and understood by the public. If data is presented without context or caveats, it can give an incomplete or misleading picture. We often see claims about migration that are based on figures which are flawed or unreliable.

We’ve taken a look at why this figure is unreliable, and why it’s difficult to accurately estimate how many people are in the UK “illegally”.

We’ve contacted Reform UK for comment.

What does ‘illegal immigrant’ mean?

There is no single definition of the term “illegal immigrant”, though it is commonly used to refer to people who are in the UK without the right to be here. Government statistics refer to people in the UK without a right to be here as “irregular migrants”, while others use the term “unauthorised migrants”.

We often see the term “illegal migrants” used to refer to people who have arrived in the UK via irregular methods (in particular, by small boat crossing). But this isn’t the only group of people in the UK without the legal right to be here.

The University of Oxford’s Migration Observatory says “unauthorised” or irregular migrants broadly belong to four categories:

• People who have overstayed their visa
• People who enter the UK without authorisation or through deception
• Rejected asylum applicants who do not leave the UK
• Children of unauthorised migrants who are born in the UK.

So not all unauthorised migrants under this definition will have always been in the UK irregularly (for example, some may have entered on legitimate visas, and then overstayed).

We also often see the term “illegal migrants” conflated with asylum seekers. While in recent years many asylum seekers have entered the UK via irregular methods, not everyone who claims asylum in the UK is necessarily here “illegally”. It’s also worth noting that the right to claim asylum in the UK is set out in law, so some estimates may not consider asylum seekers to be unauthorised migrants.

Why the 1.2 million figure is unreliable

The 1.2 million figure referenced by Reform UK appears to be based on a study published by the Pew Research Center in 2019. It estimated that the total number of unauthorised migrants in the UK in 2017 was between 800,000 and 1.2 million.

This estimate was calculated using the ‘residual method’, which essentially estimates the number of unauthorised migrants in a country by subtracting the number of authorised migrants from the total migrant population.

The Pew study compared an estimate of the number of non-EU citizens living in the UK in 2017 with an estimate of the number of non-EU citizens in the UK with a valid residence permit in the same year (authorised immigrants). It’s worth noting that because this figure was produced before the UK left the EU, EU citizens weren’t included, as at the time they did not require visas to live in the UK lawfully.

However, in a review of the Pew study, the Migration Observatory noted that this methodology excluded migrants who had been granted permanent residence (known as indefinite leave to remain) from its estimate of the number of non-EU citizens living in the UK with a valid permit. It said this, as well as other uncertainties in the study’s estimate of the UK’s non-EU resident population, “raise significant questions about the accuracy of the estimates”.

In March this year, the Pew Research Center acknowledged this issue, saying: “The ONS data on legal immigrants that we used in our report did not include those with ‘indefinite leave to remain’ (ILR), a group legally residing in the country. Including the ILR group among legal UK residents results in lower estimated total numbers of unauthorized immigrants for both the United Kingdom and Europe in 2017 and other years.”

It said that an updated estimate taking this group into account produced a new range of 700,000-900,000 unauthorised migrants in the UK in 2017 (this broad range remained the same whether or not people with pending asylum applications were included as unauthorised migrants).

It’s also important to note that the scale and nature of migration to the UK has changed substantially since 2017, meaning that the figures this estimate is based on are likely not representative of the number of migrants in the UK in 2025.

Why is measuring ‘illegal migration’ so difficult?

Estimating the actual number of people in these groups is inherently difficult, as migrants who are in the UK without the right to be here are likely to want to avoid detection by authorities.

Various studies have attempted to estimate the UK’s unauthorised migrant population, but the Migration Observatory warns all such estimates should be treated with caution, noting that “all figures are highly uncertain and have large margins of error”.

Some irregular migration is captured within official migration statistics—for example, small boat arrivals are included in the Office for National Statistics’ (ONS) long-term international migration figures.

But neither the Home Office nor the ONS currently publish estimates of the UK’s total irregular migrant population.

In a 2019 report, the ONS said: “By its very nature, it is extremely difficult to know the exact size of the illegally resident population and due to the challenges in making reliable estimates the government has not produced any official estimates since the mid-2000s.”