Home Secretary Shabana Mahmood has ordered an urgent review after a second migrant boat evaded Border Force and reached the Kent coast in as many days.
The incidents have raised concerns about the effectiveness of Border Security Command's surveillance and interception methods.
On Tuesday afternoon, 21 people were detained after a boat arrived on the beach at Samphire Hoe, Dover.
The Home Office did not confirm if more people from the boat were being sought.
The review will also examine a similar incident on Monday, when a dinghy made it to shore at Folkestone undetected – the first such incident in a year.
Uncontrolled landings of migrants are extremely rare, with the Home Office recording only one other incident, in September 2025, since it began keeping data in 2023.
The incidents are a blow to Border Security Command, which had suggested that new surveillance technology and improved resources had made such events a thing of the past.
The government has faced criticism for its handling of migrant crossings, with some calling for the Royal Navy to be deployed to help intercept migrants.
The failure to intercept two migrant boats in two days before they reached the Kent coast was “not acceptable”, Ms Mahmood said.
A government spokesperson said: "Border Security Command officers alongside Kent police have been responding to an incident in Dover. Twenty-one individuals have already been detained under a swift enforcement response.
"We are taking this incident extremely seriously and will be conducting a thorough review into how these individuals were able to evade detection," the spokesperson added.
Meanwhile, a man believed to be a migrant was found dead on a beach in northern France on Wednesday, French authorities have said.
In a translated statement, a spokesperson for the Pas-de-Calais prefecture said: "The lifeless body of a man, who appeared to be of African origin, was discovered this morning on the Neufchatel-Hardelot beach.
"The body was found by a seafarer who alerted the authorities."
A probe will examine whether the death was linked to an attempted English Channel crossing, according to French media reports.
Local outlets reported the man, thought to be in his 20s, was found hours after a small boat departed for the UK and while other migrants hoping to make the journey remained in the nearby dunes.
The discovery of the body has raised concerns about the risks faced by migrants attempting to cross the Channel.
So far this year, 17,784 migrants have arrived in the UK after crossing the Channel by boat, including 5,315 since Andy Burnham became prime minister.
The total so far for 2026 is down 45 per cent on the same point last year.
