The agency currently has a 'positive' outlook for the exchange, which unveiled its proposed takeover of Refinitiv on Thursday.

Moody's said a successful acquisition and integration of Refinitiv would result in a robust and diverse business that could improve the exchange's profile.

But the acquisition would be a "transformative and complex transaction" to manage, since it would take several years to integrate the entities, Moody's said in a statement on Friday.

Refinitiv has over twice the LSE's revenue, operates in differing business activities with a broader customer base, and has over four times the number of employees and a significantly broader geographic footprint, Moody's said.

"Based on the information available today... the most likely outcome of its review would be a one-notch downgrade of LSEG's long-term ratings to Baa1 from A3," Moody's said.