The acquisition of Scottish transport giant Stagecoach by National Express is facing an investigation by the UK's competition watchdog that will prevent either from disposing of key UK assets while the review takes place.

The Competition and Markets Authority (CMA) indicated that it seeks to "maintain the businesses in their current shape" as it starts its probe into the merger, Stagecoach said, adding that the order will delay a planned sale of parts of its inter-city coach businesses to ComfortDelGro.

READ MORE: Scott Wright: Stagecoach sale is a further blow to Scottish prestige

National Express announced in December an agreement to buy Stagecoach in an all-share deal valuing the Perth-based firm at about £445 million. The combined operation will be valued at approximately £1.9 billion.

The deal will bring together the country's biggest bus and coach operator - Stagecoach, which also operates the Megabus brand - and National Express which is best known for its long-distance UK coach services. Stagecoach said the companies believe the disposal to ComfortDelGro should allay any competition concerns that might arise from their overlapping operations.

The deal is still expected to close by the end of this year, Stagecoach added.