Labour's Tessa Jowell, who is seen as a frontrunner for the party's London mayoral candidacy, has attacked Boris Johnson over his plans to stay on as Mayor even he is elected as an MP.
Londoners need representation by their Mayor day-in day-out. Transport, homes or jobs, Londoners' challenges deserve undivided attention.
Mr Johnson announced earlier that he planned to run for Parliament in 2015 but also remain as London mayor until 2016.
Former Cabinet minister Ms Jowell has already said she is standing down from Parliament in 2015 and is believed to be targeting a run at the mayoralty.
She has previously said that “were I Mayor of London, I wouldn’t be like Boris, certainly not”.
More top news
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.