Home secretary Shabana Mahmood has ordered an "urgent review" after the Border Force failed to intercept a second migrant boat in two days before it reached the Kent coast.
Border Security Command has detained 21 people following a boat arrival on the beach of Samphire Hoe, Dover, on Tuesday afternoon, and the Home Office has not said whether more people are being sought.
It comes after a dinghy made it to shore at Folkestone on Monday morning, which was the first undetected arrival for over a year.
Chris Philp, shadow home secretary, called it a "day of despair for the Labour government".
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In a statement on Wednesday, Ms Mahmood described the undetected crossings of two small boats this week as "not acceptable" and said the Border Security Command is investigating.
"Our improved monitoring has led to uncontrolled crossings becoming incredibly rare. But even one is one too many," the home secretary said.
"That is why I have ordered an urgent review by the Border Security Command into how this happened, and what must be done to ensure it doesn't happen again."
Ms Mahmood went on to say the smugglers who arrange the illegal crossings have adopted "new tactics", taking migrants on "increasingly long and dangerous crossings" to reach UK shores.
She also said the government's efforts to stop crossings entirely "are having an impact" by "making it much harder for gangs to launch vessels" and "forcing them to change their approach".
The thousands of migrants crossing the Channel are usually brought to shore by Border Security Command or RNLI boats.
Such uncontrolled landings are extremely rare.
Before Monday, there was only one small boat arrival without an escort - in September 2025 - since the start of 2023.
Prior to that, figures on these types of incidents were compiled by the Ministry of Defence and were not routinely reported.
The Home Office has launched an urgent review by Border Security Command into this week's beach landings, the department said.
A government spokesperson said on Wednesday: "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.
"There is a thorough investigation ongoing and it would be inappropriate to comment further."
Mr Philp added: "Our border security has descended into chaos with illegal immigrants crossing at will over ever larger distances and in ever larger boats.
"This latest fiasco shows that the French are not even trying to stop the illegal immigrants' boats in French waters, instead they are escorting them here.
"What's more, there have been two uncontrolled migrant landings in a week. It is a complete free for all - the Labour government have no idea who is coming into the country.
"The only way to stop Channel crossings is to deport anyone who arrives illegally to their home country or a safe third country. That's the only way we can end this madness."
Meanwhile, a man believed to be a migrant was found dead on a beach in northern France on Wednesday, and an investigation has been launched by the French authorities, a spokesperson for the Pas-de-Calais prefecture said.
French media reports the investigation will examine whether the death was linked to an attempted crossing of the English Channel.
Local outlets said 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.
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A total of 17,784 migrants have arrived in the UK after crossing the Channel in small boats so far this year, including 5,315 since Andy Burnham became prime minister in July.
The total so far for 2026 is down 45% on the same point last year.
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