The watchdog is considering if the proposed £27.6 billion ($39bn) merger “may be expected to result in a substantial lessening of competition within any market or markets in the United Kingdom for goods or services”, it said.

Interested parties have until tomorrow (June 3) to submit comments before the CMA decides whether an in-depth assessment is needed.

AstraZeneca, a FTSE 100 company based in Cambridge, was founded in 1999 by the merger of Swedish firm Astra AB and the British Zeneca group. In 2020 it had a revenue of £18.3bn ($25.9bn) and is responsible for one of the COVID-19 vaccines. It also produces brands Nexium, Crestor and Daxas.

On May 11, AstraZeneca announced that 98.9% of voting shareholders were in favour of the merger.

American pharmaceuticals company Alexion was founded in 1992 and last year had a revenue of just over £4.3bn ($6bn). The five medicines it manufactures – including Soliris, Strensiq and Kanuma – are all used in the treatment of rare diseases.

The pharmaceutical company also announced on May 11 that its shareholders had voted for the merger.

Announcing the merger in December 2020, AstraZeneca chief executive Pascal Soriot said Alexion had brought “life-changing benefits to patients with rare diseases”, and the acquisition “allows us to enhance our presence in immunology”.

Ludwig Hantson, chief executive of Alexion, commented: “This transaction marks the start of an exciting new chapter for Alexion. We bring to AstraZeneca a strong portfolio, innovative rare disease pipeline, a talented global workforce and strong manufacturing capabilities in biologics.”