The true scale of immigration is being hidden from the public because estimates on illegal migration have been held back, a report claims.
Private Home Office assumptions on the number of people in the UK unlawfully should be released even though they are "politically embarrassing", according to the study by two former officials at the department.
They claim that internal estimates suggest illegal immigration is running at a minimum of 150,000 a year.
This would include those who remain in the country beyond the period allowed under their visas, or after asylum claims are rejected, as well as people who arrived with no legal entitlement in the first place.
The claims are detailed in a highly critical assessment by David Wood, an ex-director general of immigration enforcement, and Alasdair Palmer, a former Home Office speechwriter.
Their paper, titled The Politics Of Fantasy and published by the think-tank Civitas, fuelled allegations that calculations which could inform the debate around immigration have been "deliberately suppressed".
The report says: "The Home Office has not published any estimate of illegal immigration since 2005.
"It has produced further estimates - but for internal consumption only.
"Those estimates are partly based on the number of illegal immigrants that officials encounter as they check businesses, colleges and housing in an effort to make sure that immigration law and regulations are being followed."
Some of those in the country unlawfully will eventually return home but the net growth in the population of illegal migrants in the UK is likely to be much higher than officially published estimates suggest, according to the report.
It says it is "understandable that the Home Office should have kept those estimates to itself".
But the authors argue it is "obviously wrong" not to release the figures, adding: "Keeping them secret may save ministers from embarrassment. But it makes proper policy planning impossible."
Official statisticians have said it is impossible to accurately quantify the number of people in the country unlawfully.
In an estimate 12 years ago, a Home Office assessment put the total unauthorised migrant population living in the UK in 2001 at 430,000.
The main count used to track trends in regular immigration is net migration, which is based on a survey of passengers and measures the difference between the numbers arriving in and leaving the country for at least a year.
Net long-term international migration was running at an estimated 248,000 last year.
The report also raises questions about whether data collected at the border and from National Insurance registrations could shed more light on immigration estimates.
And it claims that the Government has great difficulty enforcing tough-sounding policies to tackle illegal immigration.
Dr David Green, director of Civitas, said: "In recent years, successive governments have shown all the signs of wanting to hide the true scale of immigration from members of the public.
"The Government has better calculations of the scale of both legal and illegal immigration but has deliberately suppressed publication."
Immigration Minister Brandon Lewis said: "Our laws are being enforced. In fact, it is harder than ever before for those with no right to be here to remain in the UK.
"Whether it is working, renting a property, opening a bank account or driving a car, we can now take tougher action on those who flout the law.
"We expect those with no basis of stay to leave voluntarily. Where they do not, we will enforce their removal. This includes foreign criminals of whom we returned more than 6,000 last year.
"At the same time, we are continuing to reform legal routes to the UK from outside Europe and will use the opportunity of leaving the EU to take control of immigration from within the EU too."