Trade unions plan to use the European directive on employment law to force buy-out firms to disclose their expected returns and improve employees' bargaining power.

The move will put pressure on private equity investment returns across Europe as employees seek a greater share of the profits generated from their work. It adds to the squeeze on the private equity industry, which is already seeing fund managers such as Deutsche Asset Management, M&G and Fidelity resisting firms' attempts to buy public companies cheaply.