Employers are being warned against complacency about a second wave of swine flu after the government revised its planning assumptions on Thursday to suggest a fresh outbreak will come later and be less severe than first thought.

The Chartered Institute of Personnel and Development and the Business Continuity Institute, in joint recommendations published on Friday, urge employers to make “worst case” contingency plans for up to half their staff being absent for between two and four weeks – more stringent than the government’s ­swine flu guidance.