Mahmood said the beach landings were “not acceptable”.
She said: “Our improved monitoring has led to uncontrolled crossings becoming incredibly rare, but even one is one too many.
“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.”
She said that new tactics adopted by people smugglers, including launching small boats from much further west in France, were “a reminder of two things”.
The home secretary said: “Firstly, these gangs have no regard for human life. They are putting people on increasingly long and dangerous crossings.
“Secondly, our efforts are having an impact. We are making it much harder for gangs to launch vessels, forcing them to change their approach, and as a result we are significantly reducing the numbers coming here by small boat.”
Early on Wednesday, migrants were brought to Dover after embarking on a crossing from a village near Bayeux in Normandy to Gosport in Hampshire, spending more than 33 hours at sea.