Under the new system, which was launched this week, workers who have worked in construction without a formal qualification will be able to convert their expertise to a Level 2 National Vocational Qualification under the guidance of on-site assessors.
Firms can invite assessors to judge whether workers' expertise is up to NVQ Level 2 standard, or whether they need top-up training.
The new scheme will help firms satisfy clients' requests for proof of employees' qualifications when tendering for contracts.
It will also offer thousands of workers whose jobs are threatened by new demands for qualifications the chance to validate their skills.
More than 4000 assessors at colleges and in-house at contractors can already award NVQs for construction.
Under the new system, CITB registered firms can claim a one-off £300 grant towards becoming in-house "assessment centres", and up to £150 for each member of staff they train as an assessor.
Employers that fulfil the CITB's grant conditions will receive a £650 grant towards the cost of training each worker up to NVQ Level 2.
Training in occupations for which there are no training facilities, such as tunnelling and demolition, roof sheeting, cladding and piling, will be provided on site by the CITB.
But bricklayers or plumbers will be expected to supplement their skills at training centres or colleges.
The plan is part of the CITB's bid to get 80% of the UK's 1.5 million construction and engineering workers qualified to NVQ Level 2 or equivalent by 2003.
CITB chairman Hugh Try said the scheme had attracted strong employer support before its launch.
"Good employers and their federations and trade associations have recognised for some time that a formally qualified and highly skilled workforce is needed for the industry to be profitable and competitive long term.
"It will not be right for every company, but where it is, it means we can now offer a real alternative for firms that have had difficulty seeing how they could achieve a properly qualified workforce."