The $29bn merger between pharmacy-benefit managers Express Scrips and Medco Health Pharmacy will "harm small business opportunities" and "stunt job growth" if it goes ahead, an advocacy group has said in a letter to US federal anti-trust regulators.

Addressing FTC Chairman Jon Leibowitz, Small Business Majority, which represents the US’s 28 million small businesses, argued that the merger could reduce competition, raising costs and potentially forcing small-business owners to drop healthcare coverage for employees.

The group is the first of a large group of employers to express opposition to the merger.

Medco and Express Scripts are two of the three largest companies that administer prescription drug plans for insurers and employers.

They voiced confidence last week over the merger, saying the deal would overcome regulatory concerns to close in the first half of next year