British police hunting those behind a bomb which injured 29 people on a London train on Friday said they had arrested an 18-year-old man in a move described as significant.

"We have made a significant arrest in our investigation this morning," Neil Basu, Senior National Co-ordinator for Counter Terrorism Policing, said in a statement. "Although we are pleased with the progress made, this investigation continues and the threat level remains at critical."

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The man was arrested under the Terrorism Act in the southern port area of Dover.