Police have warned of "serious disruption" to drivers using the M4 and M5 on Monday 4 July due to a planned protest.
Protesters intend to block the Prince of Wales Bridge from 7am until 7pm as part of a nationwide campaign against rising fuel prices.
The protest is due to start at the M4 Magor services at junction 23A eastbound and the Clevedon Interchange at junction 20 of the M5 westbound.
It is also expected to cause disruption to the M48 Severn Bridge and the M32.
Drivers are being urged to avoid the area or plan alternative routes.
Bristol Airport has issued a warning urging travellers to allow extra time if heading to or from the airport.
Police said additional officers will be brought in to ensure the protest is carried out legally.
Chief Superintendent Tom Harding said: "Gwent Police, and Avon and Somerset Police, are working jointly with neighbouring police forces and partner agencies to ensure emergency and critical services continue and to reduce disruption to both road users and local communities, however we are preparing for serious disruption throughout the day.
"I would encourage drivers to reconsider their journey, consider working from home and avoid the area where possible.
"The right to protest under UK law must be balanced with the rights of the wider community who may be affected. We have additional officers and support in place on Monday to ensure the protest is carried out in accordance with the law."
The planned protest is thought to have been organised by the Facebook group Fuel Price Stand Against Tax, and has attracted both criticism and support online.
One Facebook user commented: "Good luck to all the protesters this country is spiralling out of control and we somehow need to put the brakes on otherwise it will push a lot of people over the edge."
While another said: "The right to protest yes, but isn't it illegal to block roads and affect people going about their business, people already struggling."
One user does not feel the protests will be effective, saying: "Waste of time and resources. Will make no difference at all."
But some are hopeful, commenting: "Take my hat off to all the vehicles that are standing up for us, the cost of living is scary hope they listen to them and start dropping petrol and cost of living."
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