A further 1,400 jobs will go at other UK sites, including 713 in Rotherham, 108 at Wednesbury in the West Midlands, 93 in Scunthorpe, and 61 at Wolverhampton.
The other affected Welsh plants include Shotton, Ammanford, and Pontardulais.
And 80 jobs are to go in Scotland out of 500. Corus operates two service centres in Bellshill and Midlothian and the Dalzell plate mill in Motherwell.
Corus said it would "make every effort to achieve the job losses through voluntary redundancies".
"The structural changes we are proposing today have been carefully considered," said Corus chief executive Philippe Varin.
"They are essential for the future of the business."
Corus workers were told about the job cuts this morning.
The firm said the restructuring work would help it improve annual profits by more than £200m.
"This is a body blow for UK manufacturing," said John Wilson, senior officer of the GMB union.
Newport West MP Paul Flynn said the decision to mothball the Llanwern site was "a bitter blow for the workers and their families".
"There is virtually no alternative for blue-collar workers with skills from the steel industry. It is going to be an extremely difficult period."
And for the Tories, Shadow Work and Pensions Secretary Theresa May said: "This a bitter blow for the thousands of hard working employees at Corus."
GLOBAL JOB CUTS
Caterpillar - 20,000
ING - 7,000
Philips - 6,000
Corus - 3,500
Home Depot - 7,000
Pfizer/Wyeth - 20,000
General Motors - 2,000
Tata Steel said it would also close the main final salary pension scheme for Corus UK workers to new joiners, who will be offered a "money purchase" scheme to join instead.
"While the scheme is in a healthy position, the company will, in line with market practice, close the defined benefit scheme to new recruits, who will be offered a defined contribution scheme," the company said.
"Steps will also be taken to ensure that the company contribution to future service for existing members remains at 12%," it added.
Slump in demand
Steelmakers around the world have been hit by falling demand from carmakers, shipbuilders, construction and heavy engineering sectors, which, in turn, have seen demand for their products drop.
Our members have supported Corus through good times and bad, and now expect Corus to support them
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