The confederation has passed its figures to the Department of Trade and Industry for its consultation on the impact of losing the opt-out.
in output could cost the UK a reduction of 20 per cent in GDP - £18.6 billion.'
The confederation warns that overtime prospects would be severely reduced and that a hospital with a two-year build programme could take an extra nine months to complete.
It adds: 'Whether or not the severe delay would be socially or publicly acceptable must be borne in mind politically.'
Last week trade and industry secretary Alan Johnson won enough support from European allies, including Germany and Italy, to block compromise attempts to remove the opt-out by 2012 and allow individual countries to apply for an extension.
A confederation spokesman said: 'We welcome this news because we welcome the flexibility the opt-out gives us. We need the
Government to maintain support for its current stance.'
But the battle has been postponed rather than won. Labour
MEPs voted to scrap the opt-out in May and European officials are
likely to revisit the proposals next year.
The spokesman added: 'We are looking ahead and planning for if we eventually lose the opt-out but this delay is welcome.'
Construction unions are still bitterly opposed to the UK's plans to retain the opt-out. Ucatt general secretary Alan Ritchie dismissed the confederation's claims as 'nonsense'.
He said: 'There are construction companies working profitably in the UK and working within the 48-hour week through good
'This opt-out belongs to the Dickensian era and should have
been killed off once and for all.'