Thousands of teenagers are expected to attend an open day at Oxford University today.
The event which takes places over two days has caused traffic jams in the city in the past. The university has urged those coming to use public transport or the city's park-and-ride bus services.
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A number of teenagers, aged 13 to 15, from Portsmouth, have been treated in hospital, after taking tablets believed to be diazepam.
Thursday Evening's Pollen Forecast
Thursday night's weather for the east of the region