I am immensely honoured and proud to accept this role. From the hundreds of messages I received during the Six Nations I know what supporting England means to millions of people and I am privileged to be involved.
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The incident is not believed to be terror-related, British Transport Police said.
The royal couple have been in the US to mark the 300th anniversary of the foundings of New Orleans and San Antonio.
A small number of people were treated at Southgate Tube station, British Transport Police said.