Free crossing of the Severn bridges could increase traffic and costs to businesses, according to a hauliers' group.
Hold-ups in crossing the river will mean businesses which rely on transporting goods across the waterway lose money, Road Haulage Association (RHA) chief executive Richard Burnett said.
The tolls are being scrapped from Monday.
While Mr Burnett is in favour of abolishing crossing fees, he warned that any increase in transportation costs resulting from traffic would be passed on to customers.
He said: "We welcome the abolition of tolls as operators are already working to incredibly tight margins."
Hauliers "have no choice but to pass toll prices on to their customers," he added. "They then have to pass it on to theirs – everybody pays."
However it is "essential that traffic remains free flowing", he said, adding: "Congestion as a result of increased vehicle numbers will quickly cancel out any advantages resulting from a toll-free crossing.
"The abolition of tolls on the Severn Bridge will inevitably increase the volume of traffic on this particular route."
Gregg Griffiths, managing director of transport business Collier Haulage, based in Pontypool in South Wales, said the abolition of toll fees represents a "big saving" for his business.
He predicted the move will reduce costs to his firm by around £50,000 to £55,000 per year.
"It will go straight on the bottom line," he said.
"We probably cross it 10 times or more a day."
- Queen and Duke of Edinburgh open Severn bridges in 1966
Crossing the Severn bridges from England into South Wales in a lorry costs £16.70, with fees only applying in the westbound direction.
The Government has claimed scrapping the tolls will provide an immediate boost of more than £100 million per year to the Welsh economy.
Over the next decade this economic benefit will rise to more than £1 billion, it added.
Tolls have applied to the original Severn Crossing since 1966 and the second crossing, renamed the Prince of Wales Bridge in April this year, has always charged fees since it opened 30 years later in 1996.
The current fee for a car crossing from England into South Wales is £5.60, and regular commuters are expected to save more than £1,400 a year.
The tolls were reduced on New Year’s Day this year when the bridges returned to public ownership, although the Government initially declined to scrap the charges altogether.
The final driver to pay to cross from England to Wales will be Welsh Secretary Alun Cairns.
He said: “The end of the tolls is a major milestone for the economies of South Wales and the south-west of England and will remove historic barriers between communities.
“Scrapping the tolls means an end to generations of people paying to simply cross the border and delivering this has been one of my key aims as Welsh Secretary.
“A week before Christmas drivers will no longer have to pay every time they cross the border, meaning more money in their pockets, helping them with the cost of living and leaving them with and more cash to spend in their local areas.”
Transport Secretary Chris Grayling added: “We made a commitment in the manifesto to deliver free crossings over the Severn and that’s exactly what we’re delivering.
“This move will put £1,400 a year in the pockets of thousands of hard-working motorists and help transform the economy in the South West and South Wales, creating new opportunities and helping drive future growth.”
Earlier this year, more than 30,000 people signed a petition against the Second Severn Crossing being renamed after the Prince of Wales.
The new title, which has the agreement of the Queen and Prime Minister Theresa May, was to recognise Charles’ 70th birthday year and 60 years since he became the Prince of Wales.