Employers are failing to prepare for the "challenge" of age discrimination laws due to come into force in October, the conciliation service Acas has said.
Older workers may be able to work beyond the age of 65
The laws will ban direct and indirect discrimination in the areas of recruitment, promotion and training.
But an estimated four out of every five companies have so far failed to inform or train staff, according to Acas.
The service is publishing guidelines it says will help eliminate "outdated prejudices" about age.
Chairwoman Rita Donaghy urged employers to embrace the changes.
"This is an excellent opportunity for employers to go beyond compliance and increase their productivity," she said.
From 1 October employers will be banned from forcing workers to retire before the age of 65.
And bosses will have to give an individual at least six months notice of their retirement date.
Workers will also be able to remain in their job beyond the age of 65 if their employer agrees.
The changes have been broadly welcomed by employers' groups and trade unions, with the TUC saying they would "make a world of difference".