More than 300 jobs are being lost at plants in the West Midlands, with other losses at sites in Hartlepool, Wrexham and Tredegar, administrator PWC said.
Unions blame cheap steel imports for Caparo's financial problems.
A number of sites have closed with immediate effect and more jobs are expected to go in the coming weeks.
A skeleton staff will remain at some of the sites to shut them down over a period understood to be a matter of weeks, before they also lose their jobs.
Vehicle technology divisions Caparo Forging Europe at Dudley, in the West Midlands, and Hartlepool in Teesside and the Caparo Atlas Fastenings business at Darlaston, also in the West Midlands, have shut.
Updates on job loss announcements
Also closing down is the steel products division Fairbright Wire and Caparo Tubes in Tredegar, Blaenau Gwent.
Thousands of job losses have been announced in the steel sector in recent weeks, with the collapse of SSI in Redcar, Cleveland, and cutbacks at Tata Steel in North Lincolnshire and in Lanarkshire.
Paul Wood, who has worked at Caparo Forging, in Netherton, Dudley, for nine years, said he was called to a meeting on Friday morning and told he would lose his job.
He said he was "choked".
The Netherton foundry has been there for a century and makes gears for Volvo lorries and Scania lorries.
Joe Morgan, GMB Birmingham and West Midlands regional secretary, said: "We are still of the opinion that Caparo can be saved."
Unite West Midlands regional secretary Gerard Coyne said the closure of the Darlaston factory was "another grim day" for the manufacturing community in the Wolverhampton area.
Government has published new guidelines for buying steel for its major projects to help British suppliers compete on a level playing field with international suppliers.
Matthew Hancock, Paymaster General and Chair of the Working Group, said: "I don't want contracts going abroad if the best bid is a British bid with all the social and economic benefits that brings."