Tougher enforcement fines are being brought in to stop drivers using the Rotherhithe Tunnel in vehicles that are too big or too heavy.
Vans and lorries could mean £130 every time they are driven through the A101 Thames tunnel link from the Commercial Road in Limehouse down to Rotherhithe from December 2, drivers are warned.
The ageing ventilation system can't cope if a large vehicle catches fire driving through and fills the tunnel with smoke and fumes.
Transport bosses at City Hall have had to get tough because of "persistent offenders" breaching the current restrictions on vehicle height, width and weight, it has emerged.
"The restrictions are absolutely essential for safety," Transport for London's network director Glynn Barton insists.
"People driving vehicles through the tunnel who don't comply with these restrictions are putting themselves and others at risk."
TfL is bringing in separate fines for each journey to stop vehicles being driven through the tunnel time and again that are more than 6ft 6ins wide or high, or goods vehicles weighing more than two tonnes.
The restriction enforcement has been carried out by number-plate recognition cameras since February, checked against the national databank of registered vehicles to trace if they are prohibited.
Letters were resent to registered drivers caught breaking the rules rather than issuing fines for the first two weeks, for "a chance to get used to the new restrictions", then discretion was used to limit prohibited vehicles breaching the regulations to one fine a day.
But now fines for each journey start from December 2 because of "continued repeat offences" of prohibited vehicles being driven through. Letters have also now gone out to van hire firms warning about the tougher penalties.
The Rotherhithe Tunnel was first opened in 1908. But the limited ventilation can no longer cope with today's traffic volume and larger vans and lorries compared to a century ago.
Upgrading the system is now being planned, the work likely to start in the New Year.
A barrier has already been installed on the approach to the north entrance, just off Commercial Road, with a diverting slip road for any approaching vehicle too wide to get through, but doesn't restrict heights. An overhead gantry would have to be installed.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here