The A41 is closed this morning (October 4), while emergency services deal with an 'ongoing incident', Thames Valley Police confirms.
Between the Tring Hill Entry Slip-road and the Aston Road exit in Aylesbury the road is closed to traffic going in both directions.
The authorities are asking motorists to use alternative routes, while the incident is being addressed.
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The closure is expected to remain in place for several hours.
A Thames Valley Police spokesperson said: "The A41 bypass is currently closed from the Tring Hill Entry Slip-road, to the Aylesbury Aston Clinton Road exit. This is due to an ongoing incident. This will remain in place for several hours. Please avoid the area and plan an alternative route."