BOSSES have warned drivers that they risk being liable for the £12.50 Ulez fee if they ignore official diversion signs in an attempt to find a shortcut.
The M25, between junctions 9 and 10, has been closed both ways this weekend while concrete beams for a new bridge are installed.
This has forced motorists to follow a 19-mile diversion on A roads, crossing from Surrey into London's Ulez area.
The National Highways have assured drivers that "no enforcement action would take place," as long as they stick to the diverted route.
Motorists who ignore the diversion risk having to pay the daily Ulez fee.
Failure to do so when required can lead to a £180 fine, reduced to £90 if paid within 14 days.
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On Saturday, National Highways said they had seen an increasing number of motorists choosing to ignore the diversion route, starting at junction 8, and instead continuing on the motorway to the next junction.
Simon Elliott, National Highways senior project manager, said it was "important that drivers follow the diversion route in place".
"It is the most effective way for people travelling clockwise along the M25 to reach their destination and will help keep traffic moving as well as possible during this weekend's closure," he added.
"Coming off at junction 9 means non-freight drivers will end up travelling behind or alongside HGVs which is something we are actively trying to avoid and is likely to have a negative impact on travel times, congestion and the communities they pass through."
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Reports suggest construction over the weekend has made good progress.
Several of the beams that will form the new bridge have been lifted into place.
The 19-mile diversion route saw an "unusually low volume of traffic", with residents and local businesses reporting quiet roads and reduced footfall in their towns and villages.
Councillor Rod Ashford, from the Reigate and Banstead Borough Council, said: "Given the A217 would be a route to divert M25 traffic to Epsom and around (to the) M25 junction 9, traffic in my area is quiet."
Workers in Epsom, a town on the diversion rout claimed there had been a "downturn in business and walk-in trade" due to the works.
National Highways previously urged drivers only to travel the route if necessary, to avoid the closure and to complete their journeys in other ways if possible.
The M25 closure is the second of five planned weekend closures which come as part of a £317 million project to improve junction 10.
It comes after villagers slammed the first planned daytime closure of the M25 as a "nightmare".
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The Diversion Routes
Diversion route clockwise for all but over-height vehicles:
- Leave M25 at junction 8, A217 (Reigate).
- Follow the A217 London, Sutton, (A240) Kingston.
- After 3½ miles turn left onto the A240 Epsom, Kingston.
- After 3 miles at the Esso roundabout, turn right onto the A24 (A240) Kingston.
- Continue for 3 miles and turn left onto the A3 Portsmouth, Guildford.
- Continue for 9½ miles to the M25 and re-join the motorway at junction 10.
Diversion route anticlockwise for all but over-height vehicles:
- Leave M25 at junction 10 to join the A3 towards London.
- After 9½ miles at the Hook interchange leave the A3 turning right onto the A240 Epsom, Reigate.
- Continue for 3 miles to the Esso roundabout and turn left onto the A240 Reigate.
- Continue for 3 miles then turn right onto the A217 Reigate, M25.
- After 3½ miles turn left to re-join the M25 motorway at junction 8.