Two men have been arrested on suspicion of arranging the "illegal movement of migrants" across the English Channel during December.
The pair - an Iranian and a Briton - were arrested in Manchester.
More than 230 people have reached the UK in small boats since November.
The UK is due to redeploy two Border Force patrol boats to the Channel from the Mediterranean, where they have been helping with the thousands of people who have tried to cross to Europe.
The 33-year-old Iranian and 24-year-old British man were arrested by the National Crime Agency in Pendleton, Greater Manchester, on Wednesday.
The UK Border Force currently has two coastal patrol vessels in the Channel as well as a cutter, HMC Vigilant, which can rescue several boatloads of people at once.
Home Secretary Sajid Javid says two of the three Border Force vessels currently in the Med - HMC Seeker and Protector - will return.
But the Home Office would not give further "operational" details of where they were or when that might happen.
Mr Javid is also reported to have requested help from the Royal Navy in the Channel - offshore patrol vessel HMS Mersey could be used.
A total of 239 people are known to have reached the UK in small boats since November, including 12 migrants found off the Kent coast last week.
The Ministry of Defence said the armed forces were "ready to provide additional capacity and expertise".
It comes after Mr Javid sparked controversy during a visit to Border Force staff in Dover on Wednesday by questioning whether those risking their lives to cross the English Channel in small boats were "genuine" refugees.
The home secretary also defended escalating the UK's response to the crossings, saying 80% of the 539 people who had attempted to make the crossing in small boats in 2018 had done so since October.
Sorry, your browser cannot display this map
More widely, figures released in May showed an estimated 2,366 people entered the UK "clandestinely" in 2016-17 - on boats, via ferries or on lorries. The estimate for 2017-18 was 1,832.
It is not known how many of those who entered the country in this way went on to apply for asylum in the UK.
In 2017, there were 26,350 asylum applications in total. A decision was made in 21,290 of those cases, with 31.8% gaining some form of protection.
During the same period, there were 198,255 applications for asylum made in Germany, 126,550 in Italy and 91,070 in France.
'Lost control of borders'
Lib Dem spokesman for home affairs, Sir Ed Davey, said the government had "lost control of our borders" because of cuts of 600 border staff since 2015.
He told BBC Radio 4's Today programme: "Rather than taking the blame for that, they're scape-goating the most vulnerable people, blaming the refugees. That's just moral cowardice."
Safe routes such as the refugee family reunion scheme and vulnerable persons resettlement scheme should be used, he said.
Conservative MP Anne-Marie Trevelyan said France could look "just as well" as the UK at a "genuine" refugee's case and said she had asked Mr Javid to investigate how the UK could "help the French to manage this more".
But she added: "That Channel waterway is the most congested piece of waterway in the world.
"People getting into small boats are incredibly likely to lose their lives. Nobody wants to see that happen."
French police said they stopped 14 migrants attempting to cross the Channel from Boulogne on Tuesday - the latest in a growing number of people intercepted by the French authorities in the past month.
The government has agreed a joint action plan with France, which includes the redeployment of the two Border Force vessels from the Mediterranean.
Only one of the five Border Force cutters had been working in the Straits of Dover.
The two being brought back have been taking part in Operation Frontex, the pan-European effort to deal with much larger migration flows from North Africa and the Middle East, to Italy and Greece.