An investigation has been launched following reports that a small boat carrying migrants reached the Kent coast without being intercepted by Home Office officials.
People crossing the English Channel are usually brought to shore in Border Force or RNLI boats.
However one vessel reportedly slipped through without detection on Saturday, when 1,096 people were recorded having made the journey.
It would be the first time this has happened since December 2022. Just under 100,000 people have made the perilous journey since that date.
The news, first reported by Bloomberg, came as Border Force vessels and RNLI lifeboats responded to more small boats on Tuesday.
One woman died during a rescue operation in the Channel, police said.
In a statement, Kent Police said it was "made aware by the Coastguard of an incident in the channel involving a small boat just after 1pm" on Tuesday.
One woman was airlifted back to shore where she was pronounced dead, a spokesman for the force said.
Police said five people were detained in a village north of Dover on Saturday and were passed to immigration authorities.
No small boat arrivals to the UK coast without an escort have been officially recorded since December 10, 2022.
A Kent Police spokeswoman said: “At 7.50pm on Saturday September 6, Kent Police was called to a report of suspected migrants near Kingsdown, Deal.
“Patrols attended and five people were detained and passed to Border Force officers.”
The Home Office declined to comment as the investigation is ongoing.
The probe comes as more migrants continued to cross the Channel on Tuesday, with at least five boats seen arriving in Dover.
Footage shows people arriving at the Kent port, and disembarking from Border Force boats.
The crossings comes as the Prime Minister’s new cabinet met for the first time on Tuesday, including newly-appointed Home Secretary Shabana Mahmood who is tasked with ramping up the UK’s response to the record arrivals.
Since starting her role just days ago, the number of migrants arriving in the UK after crossing the English Channel topped 30,000 for the year so far.
It is the earliest point in a calendar year at which the 30,000 mark has been passed since data on Channel crossings was first reported in 2018.
Ms Mahmood said the numbers were “utterly unacceptable” and that she expected migrant returns under a deal agreed last month with France to begin “imminently”.
The “one in, one out” agreement with France detained the first migrants under the scheme on August 6, but has not yet seen sent anyone back to the continent.
Asked why no returns have taken place so far, the Sir Keir Starmer’s official spokesman said: “I’m not going to get into the operational detail of it but, as I say, we’ve said that we’re expecting returns to begin this month.”
He added that the political turmoil in France was not to blame for the length of time the process has taken.
