More than 27,000 people suspected of illegally entering the UK have been arrested over the past three years, figures obtained by the BBC show.
The statistics were complied from responses to Freedom of Information Act requests to 39 police forces in England, Wales and Northern Ireland.
Many of those arrested were found at motorway service stations and truck stops, having hidden in lorries.
The Home Office said it wanted “long-term solutions” to illegal migration.
Commons Home Affairs Committee chairman Keith Vaz called for “urgent action” to be taken to tackle the problem.
BBC home affairs correspondent Danny Shaw says the figures are the most comprehensive yet to be compiled, and highlight the increasing burden placed on police by migrants who have slipped through border controls unnoticed.
In 2013, officers made 7,700 arrests for illegal entry. The number increased slightly the following year, and rose to 9,600 in 2015, when many countries in Europe were struggling to deal with the refugee crisis.
The total over the entire three-year period, including the first quarter of this year, was 27,800.
This number does not include people arrested for staying longer in the country than their visa entitles them to, nor those detained at ports and airports, who are dealt with by Border Force staff.
Immigration offenders can be those who have entered the country clandestinely, acted in breach of their conditions, failed to leave at the end of their visa or used deception to gain entry.
Mr Vaz told the BBC the “astonishing” figures “basically show[ed] that our border is not secure”.
“The government keeps maintaining that they have got water-tight security at our borders,” he said.
“If 27,000 people have been arrested for entering the country illegally by our police forces, then it shows that this problem is even worse than we had anticipated and we expect urgent action to be taken.”
Mr Vaz said the problem meant “greater pressure” on the UK’s police forces, as they were “having to do the work of border security”. More resources were needed, he added.
The Home Office said where someone is found to have no right to remain in the UK, it will take action to remove them.
A spokesman said: “As part of the ongoing action we are taking to secure our borders, we have invested tens of millions of pounds to bolster security at ports in northern France.
“We are also committed to finding long-term solutions to the problem of illegal migration, which is why we created the Organised Immigration Crime Taskforce last year to work with law enforcement and international partners to target the organised crime gangs behind people smuggling.”
There were “clear signs” that the action was working, he added. (bbcnews)