Watch ice falling onto the old Severn bridge, from an overhead cable. Footage courtesy of Severn River Crossing Plc
One Severn bridge has reopened but the other remains closed after falling ice smashed the windscreens of six cars.
One lane has opened in both directions on the M48 crossing, although a speed limit of 50mph is in place.
The second Severn crossing and M4 remain shut after metre-sized sheets of ice fell from overhead signs. Three cars were damaged on each bridge.
The closures at 1000 GMT led to major traffic disruption with tailbacks of traffic diverted off the M4.
The Highways Agency said the second toll bridge will remain closed for health and safety reasons.
It is the first time it has been forced to shut for weather reasons since it opened in 1996.
Traffic Wales said it was working with Gwent and Avon and Somerset police forces, the south Wales trunk road agency, the south west regional control centre and Traffic Link "to deal with this situation as efficiently and effectively as possible".
Earlier, National Express turned around coaches and suspended services east of Swansea.
Motorists wanting to use the Severn crossings eastbound had been diverted off the M4 via junction 24 on the A449 Raglan following through to the A40 Monmouth, to join the M50 at Ross-on-Wye then to use the and M5.
SEVERN BRIDGE FACTFILE
This is the first time the Second Severn Crossing has closed because of the weather
Both bridges have been closed at the same time once before - due to a chemical incident at Avonmouth
The second bridge took four years to build and opened in 1996
The original bridge, which now carries the M48, was opened in 1966 by the Queen
Traffic Wales information line on 0845 602 602 0
Traffic was in a tailback at High Cross, the other side of Newport.
Westbound M4 traffic was also being diverted via the M5, M50 and Ross and Monmouth.
Jim Clune, general manager of Severn River Crossing plc, said sheets of ice up to a metre square had fallen from overhead sign gantries and smashed the windscreens of the vehicles.
He said: "The situation is that bridge closures have been requested because there were reports of sheets of ice falling from overhead sign gantries and smashing car windscreens.
"It's very much a safety hazard and of course these gantries span all carriageways on the motorways.
The scene on both sides of the bridge - footage of ice falling from bridge courtesy Severn River Crossing Plc
There were no reports of injuries.
Mr Clune said the incident was "very unusual" and said it was the first time the Second Severn Crossing, which opened in 1996, had closed because of the weather.
Both bridges have been closed at the same time once before - due to a chemical incident at Avonmouth.
"We are in constant contact with the Highways Agency to see what we can do to alleviate the situation as soon as possible," Mr Clune said.
South Wales chamber of commerce managing director David Russ said the bridges' closure showed how vulnerable the region's economy was to problems on the M4.
He said: "If you include today this is the fourth or fifth time the M4 has been closed in the last two years.
"The whole place gets locked up and you can't move. For businesses getting goods out of here it becomes a nightmare, frankly.
"At the moment we are just at ransom when the M4 closes."
Meanwhile, the M4 between Junctions 22 and 25 is described as "hazardous". One lane is closed in both directions on that stretch of the motorway and a 30mph speed limit is in place.
Drivers caught in the closure of the two Severn crossings describe their feelings.
Four inches of snow fell in Powys and the A470 is shut between Merthyr and Brecon. Drivers are urged to take care.
The A4061 at Treorchy and Hirwaun is also closed as is the A4067 Sennybridge to Ystradgynlais road.
Police said the A465 Heads of the Valley Road is "just passable" in both directions between A469 (Rhymney) and A4060 (Dowlais Roundabout).
First Cymru warned of disruption to bus services, particularly in parts of south Wales.
In Swansea, it said there will be services on main roads only first thing and not including hilly areas.
It said it was unable to operate to Glyncorrwg, Blaengwynfi, Cymer or Croeserw in the Afan Valley, and services from Port Talbot were going only as far as Cwmavon.
Flights were delayed for about two to three hours at Cardiff Airport earlier while the runway was cleared but the backlog is gradually clearing.
Cardiff is also receiving a number of diverted flights from other airports, mainly easyJet planes which were unable to land at snow-bound Bristol Airport.
However, staff say they are not expecting anticipate any further delays or problems.
More than 300 schools are closed across Wales, around half the numbers which have been shutting this week.
Snow has also affected Powys, parts of south east Wales and the heads of the valleys
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