The 40th anniversary of the opening of the first Severn crossing is being celebrated in England and Wales.
Construction took over three years Pic: Severn River Crossing PLC
The mile-long £8m Severn Bridge was opened to traffic by the Queen on 8 September 1966.
The bridge was built to replace the ferry service that crossed from Aust Cliff in South Gloucestershire to Beachley Peninsula in Gloucestershire.
Severn Bridge PLC is marking the occasion by opening one of the towers to 150 members of the public.
Each place is being sold for £40 and the money raised will go to St Peter's Hospice, Bristol, and St Anne's Hospice, Newport.
The charities are hoping to raise £6,000 over the two days the tower will be open.
SEVERN BRIDGE FACTS
The bridge is 1,597m (5,240ft) in length
Its two towers stretch approx 122m (400ft) above the river
The sound of lasers in the film Star Wars was made by striking one of the bridge's suspension wires
Bridge has carried more than 300 million vehicles since it opened
The Severn Bridge, which took three-and-a-half years to build is actually comprised of two bridges - the main section spanning the Severn and the other crossing the River Wye.
The main span alone contains 18,500 tons of steel in the construction.
People queued for days to be the first across the bridge and more than 100,000 cars crossed in the first few days it was open.
Richard Parnaby, professor of architecture at the University of the West of England, Bristol, said he used the bridge every day on his journey to and from Barry - even though it was shorter to use the Severn Crossing.
"I love the old bridge because it's such an elegant and simple idea and at the time it was built it was really pushing the technology of such building.
The crossing is one mile long Pic: Severn River Crossing PLC
"It was the longest single span bridge of the time and it sits so well in the landscape," said Mr Parnaby, who is also commissioner for the Design Commission Wales.
"When you drive across it you get fantastic views of the estuary because you're up so high. It has stood the test of time extremely well - it's an iconic structure.
The Severn Bridge carries two two-lane carriageways, a footpath and cycle track.
It was strengthened in the 1980s to cope with the growth in volume of traffic, but was replaced as the main route into south Wales in 1996, when the second Severn Crossing was opened downstream.