Motorists could face major disruption on Monday amid widespread protests at rising petrol and diesel costs and calls for a cut in fuel duty.
Organisers are expected to block the Prince of Wales Bridge crossing between England and Wales, according to police.
Other major roads could also be affected, campaign group FairFuelUK said.
The organisation is not involved in the action but founder Howard Cox said he is “fully supportive” of the demonstrations so long as they are conducted legally.
The protests are understood to be organised via social media under the banner Fuel Price Stand Against Tax.
Mr Cox said he believes the protests will target mainly three-lane motorways and see slow-downs on two lanes, leaving the fast lane clear for traffic to pass.
He told the PA news agency: “I totally support their protest because people have reached the end of their tethers at the moment.”
He said other countries had cut fuel duty by more than the UK and asked “why the hell are we not doing it here?”
Figures from data firm Experian show the average price of a litre of petrol at UK forecourts reached a new high of 191.4p on Thursday, while diesel rose to 199.1p.
Last week, Chancellor Rishi Sunak told MPs he will carefully consider calls for a “more substantial” fuel duty cut after the 5p per litre reduction implemented in March failed to halt price rises.
Mr Cox called for a cut of at least 20p, and warned that protests will continue if not.
He said: “There is an appetite (for such protest). If the Government don’t actually deliver on this, I think there’s going to be some serious escalation of protests.”
Gwent Police said protest is expected to take place on the road network between 7am and 7pm on Monday.
They said organisers had indicated an intention to block the Prince of Wales Bridge, with the protest starting on the M4 at Magor services, junction 23A eastbound, and junction 20 of the M4 westbound.
Bristol Airport advised travellers to allow extra time for their journeys.
In a tweet, the airport said: “Please note that there is a planned fuel protest to block the River Severn Bridge crossings this Monday July 4 from 8.30am.
“The protest will likely affect the M5, M4 and the two crossings to Wales. Please allow extra time if travelling to or from the airport.”