Britain's competition regulator said on Tuesday it has started a probe into AstraZeneca's $39 billion takeover of US drugmaker Alexion for possibly reducing competition in the UK market or elsewhere.

The Competition and Markets Authority said it was inviting comments from any interested party on the deal to help with its assessment. It has set a deadline of June 3 for the insight from any stakeholders.

Anglo-Swedish drugmaker AstraZeneca agreed to buy Alexion in December in its largest ever deal, diversifying away from its fast-growing cancer business in a bet on rare-disease and immunology drugs.