Birmingham's main city centre tunnels will close later for six weeks to allow the first major work in their 40-year history to be carried out.
The St Chad's and Queensway tunnels, on the A38 between St Chad's Cathedral and The Mailbox, will shut at 22:00 BST.
Steve Brittan, Birmingham Chamber of Commerce's president, has warned it will cause "chaos" in the city centre.
The tunnels, which are due to reopen at 06:00 on 2 September, will also shut for six weeks for more work next year.
Over the next six weeks, nearly 1,000 new lights will be installed in the tunnels and about 21,000 sq m of fire protection will be added to the walls and ceiling of the tunnels.
Engineers said they will also improve fire escapes and the general appearance of the tunnels.
Amey, which is carrying out the work on behalf of Birmingham City Council, said it looked at various options of how to carry out the work and that shutting the tunnels completely for six weeks was the cheapest.
It said that traffic levels were usually up to 20% lighter during the school summer holidays, which the work has been organised to coincide with.
The tunnels have already been shut for four weeks overnight and will close during the night again for two weeks after the complete closure.
Mr Brittan said having the two tunnels closed for the full six weeks would make traffic "horrendous".
He said he would boycott the city centre while the work was being carried out and urged other drivers to do the same to alleviate congestion problems.
Along with road diversions, extra measures have been put in place by Amey, National Express and Centro to encourage drivers not to come into the city centre.
A 500-space park-and-ride site has been set up at Birmingham City University's Perry Barr campus, extra buses are being put on and three "bike trains" are being set up to allow people to cycle in large groups into the city.
The refurbishment of the tunnels is being paid for as part of the city council's 25-year £2.7bn contract with Amey to maintain Birmingham's roads, footpaths, bridges, tunnels, street lighting and traffic control systems.
Amey has not said exactly what proportion of that is being spent on the tunnels.
Next year, work will concentrate on electrics within the tunnels.
The council and Amey have set up a website giving details of the tunnel closures.