Braving rain and an extremely muddy pitch, the Prime Minister - who said he used to play the position of hooker - passed a rugby ball around with children who were having a taster session at Millwall Rugby Club in east London.
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Three people have been arrested and charged in connection with violence between West Ham and Chelsea fans at the former Olympic stadium.
This year's poppy appeal is giving special emphasis to the plight of younger service veterans.
West Ham fans clashed with Chelsea supporters on Wednesday during an EFL Cup tie, a game which the Hammers won 2-1.