The Parsons Green bombing suspectwas arrested by chance when two unarmed officers spotted him at the Port of Dover as he tried to flee the country, it has emerged.

The 18-year-old, who is suspected of placing the powerful device on a rush hour tube train on Friday morning, was detained by Kent police as he tried to purchase a ferry ticket to Calais.

Hours after his arrest, which Scotland Yard described as “significant”, a second suspect was detained in Hounslow, west London.