The closure could be in place for several days as emergency gas repairs take place.
A spokesman for gas distribution company SGN told Bucks Radio:
"We have unfortunately had to temporarily close the A418 between Aylesbury and Leighton Buzzard to undertake emergency repair works.
We are sorry for the inconvenience caused. This closure has been implemented to ensure the safety of everyone around the worksite while these essential works are carried out.
A signed diversion route is in place for road users. We hope to be able to lift this temporary road closure, and introduce temporary traffic lights, later this week".

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