A Government spokesman said:
We are currently experiencing temporary IT problems which may add to the time taken to conduct passport checks.We are working to rectify this issue and are providing extra staff to get passengers through the controls as quickly as possible. Our priority remains security of the border.We apologise for any additional time this adds to passengers' journeys.
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The Tooting MP has been the frontrunner in the counting process ahead of Tory challenger Zac Goldsmith.
It's not just London where elections have been taking place today. There were council elections taking place across the home counties.
A list of results for London Mayor and the London Assembly