Three Channel migrants were missing, feared dead, on Tuesday night in what would be the worst small boat tragedy off the English coast since the crisis escalated in 2019.
Two migrants were saved from a dinghy in the middle of the North Sea but they told rescuers three people had gone missing overboard during their 50-mile, 48-hour journey from Calais.
The five are believed to have set off from a beach near Calais on Saturday night before being driven off course by heavy sea swells and winds, ending up 35 miles from Harwich, on the Essex coast.
The two who were picked up told rescuers that two migrants were lost on Saturday. An air and sea search was under way for the third, said to have gone overboard on Tuesday.
If confirmed, it would be the biggest loss of life off the coast of England since the Channel migrant crisis exploded in 2019, and follows two known tragedies in French waters.
A 27-year-old Eritrean man died in August after a boat carrying 36 people began to sink off the coast of France.
Five members of Kurdish-Iranian family, including a 15-month-old boy died in October last year after their boat capsized in French waters.
A government source said it underlined the need for France to stop migrants departing from their shores, while MPs said it demonstrated the "life or death" risks of the cross-Channel journey.
"It highlights the need for the French to prevent these boats leaving. That's the only way to stop this," said the source.
Tim Loughton, a member of the home affairs committee, said: "This must be a strong warning that this is not the right way to cross the Channel. It is a life and death experience and people should not be doing it."
Natalie Elphicke, the Dover MP, said: "I have long been warning of the risk of more deaths this winter. This latest incident underlines the dangers of these illegal journeys. That's why the French must do more to stop boats leaving their coast in the first place in order to save lives."
It is understood the rescue began at 4pm on Monday with Border Force, HM Coastguard and RNLI deployed through to Tuesday on a search for the migrants.
A twin-engined HM Coastguard Beech King plane carried out sweeps of the search site in the North Sea between Felixstowe and Ostend, in Belgium, and a "specialist" branch of Border Force in Dover is understood to have driven to Harwich to aid the search on Monday.
A Home Office spokesman said: "While the investigation into this incident continues, it is a reminder of the extreme dangers of crossing the Channel in small boats and the callous disregard for life shown by the criminal gangs. We are determined to do everything we can to prevent people dying in the Channel."
Bad weather has hampered attempts by migrants to cross the Channel since the weekend before last, when more than 800 reached the UK in three days. There is increasing desperation to get to Britain before the winter closes in, with 2,500 people crossing so far in October – five times the rate for the whole of the same month last year.
More than 19,500 people have crossed the Channel so far this year, compared with 8,410 for the whole of 2020.