840 illegal entry arrests by Welsh police since 2013
Police have arrested 840 people suspected of illegally entering Wales over the past three years, new figures show.
South Wales Police made the most arrests with 572 - the 14th highest in the UK.
Gwent Police saw the biggest rise in Wales with 184 arrests from 2013 to 2015, an increase of 73%.
The Home Office said it wanted "long-term solutions" to illegal migration and its strategies were working.
More than 27,000 suspected illegal immigrants were arrested by forces across the UK during the same period.
Many of those detained were found at motorway service stations and truck stops, having hidden in lorries.
Three of the four Welsh police forces responded to the BBC's Freedom of Information request.
In the Dyfed-Powys force area, there were 84 arrests - up 14% over the three years. North Wales Police did not provide a full response.
'It will get worse'
Gwent Police and Crime Commissioner, Jeff Cuthbert, said the numbers in Wales were quite small but admitted the problem would "probably get worse".
"We've seen an enormous amount of people fleeing persecution and terror. It's very important we have a mix of planning and compassion," he said.
"I am very concerned about the impact of modern slavery and people trafficking on Gwent.
"We have seen some very high-profile cases, with many illegal immigrants forced into prostitution, particularly in Newport."
He said many people were entering Wales via the Severn crossings, and border force and police needed to be better funded to tackle the problem.
"We have to make sure public services are resourced - and the cutbacks are not helping," he added.
"It's going to get harder. We have to cooperate with other European countries."
The Home Office said it would take action to remove anyone who did not have the right to remain in the UK.
A spokesman said it was committed to finding long-term solutions to tackle illegal migration and there were "clear signs" its Organised Immigration Crime Taskforce, created last year to work with law enforcement and international partners to target organised gangs being people smuggling, was working.