About 340 jobs could go in Scunthorpe, 90 in Workington and 40 in Teesside.
The firm blamed the UK market for construction steel, which is about half of 2007 levels and the burden of environmental and energy regulations threatening higher costs.
While construction is seeing a recovery in activity this is being driven mainly by housing and the growth of multi-storey apartments blocks which has favoured the concrete industry.
Commercial orders are still down and not forecast to recover until next year.
Jon Bolton, Director of the Long Products operations, said: “UK demand for construction steel has fallen further since we launched an improvement programme at our Long Products business in 2011.
“This further market decline means we must now build on the work we have been doing to restore Long Products’ ability to compete throughout the economic cycle.
“The proposed changes at our Workington engineering operations, meanwhile, are as a result of a shortfall in external projects.
“Today’s proposals would help us build a more sustainable business. This action, unfortunately, cannot wait if we are to remain a vital foundation industry supporting the UK’s manufacturing and construction sectors.
Michael Leahy, General Secretary of Community and Chair of the UK Steel unions’ Committee said he was seeking an urgent meeting with the company to get their agreement to extending the consultation period to allow for all alternatives to be explored.