Unite the Union will today tell MPs that a potential bidding war for confectioner Cadbury will further undermine workers' rights.
US-based Kraft Foods has until 2 February to persuade Cadbury investors to accept its £10.5bn hostile takeover bid.
Cadbury is due to make a statement today. Rivals Hershey's and Ferrero have until 23 January to submit a firm offer, reports Reuters.
Unite secretary general Jack Dromey said: "UK and Irish workers and UK businesses certainly do not benefit, and will continue to lose out if bidding wars are allowed to undermine the future of our companies."
The union claims that, were Kraft to buy Cadbury, it would lay off large numbers of employees and restructure the company to cut costs.
Dromey is to speak before the Business, Innovation and Skills Select Committee at the House of Commons today.