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Pressure group Marketers for an Open Web has told the CMA the project could hit smaller websites' revenues by up to 75 per cent.
The CMA launched its probe on January 8, but The Mail on Sunday understands its investigation could take two years or more.
Officials fear any changes to the browser could cause irreversible damage to competitors before the probe concludes.
The watchdog has the power to stop the rollout if it can prove that urgent action is needed to prevent significant harm to individuals or businesses in the UK. The CMA can also act if it is in the public interest.