Motorists on the M4 in south Wales face nightly closures of the Brynglas tunnels in Newport over the next eight months for maintenance work.
The closures from 20:00 to 06:00 BST from 20 June are intended to affect traffic in just one direction, although at times both tunnels may be closed.
The A48 Southern Distributor Road will be promoted as the diversion.
Economy and Infrastructure Secretary Ken Skates said there was an "ongoing commitment" to improving the motorway.
The work is due to be carried out mainly at night until February 2018, with the M4 scheduled to be closed between junctions 25A (Cwmbran) and 26 (Malpas) up to five nights a week.
Diversions will be put in place between junctions 24 (Coldra) and 28 (Tredegar Park) for through traffic, although local traffic will be allowed to travel up to junctions 25A and 26 to access local routes.
'Minimal possible disruption'
"The M4 is of vital importance to the Welsh economy and this maintenance to the Brynglas Tunnels forms part of our ongoing commitment to improving the motorway," said Mr Skates.
"The timing of this work is designed to ensure that it's carried out safely and as quickly as reasonably possible, with minimal possible disruption to road users.
"Whilst such work will always be necessary, it's clear that improvements to the current M4 alone will not resolve the wider, long term issues surrounding the congestion on the M4 around Newport.
"I'd like to be very clear that tackling these wider long term issues remains a top priority and I hope to be in a position to make an announcement to this end soon."
The Welsh Government wants to build a new £1.1bn motorway south of Newport to relieve pressure on the existing M4, but Labour ministers face opposition from other parties, some Labour AMs and green organisations on the financial and environmental cost.