Rail passengers travelling between Preston and Scotland could face higher fares when First Trenitalia take over from Virgin Trains, the competition watchdog has warned.

The Competition and Markets Authority (CMA) raised concerns over the impact of the joint venture between Aberdeen-based FirstGroup and Italian-owned train operator Trenitalia on the West Coast Main Line.

An initial investigation found that on 17 routes, passengers will only be able to choose between First Trenitalia and FirstGroup-owned TransPennine Express, with four more routes where there will only be a choice between the same two operators plus one other.

The CMA said First Trenitalia has the opportunity to offer methods to address its concerns over the West Coast Partnership franchise.

In previous cases, such as the East Midlands Rail and South Western franchises, the watchdog was satisfied by companies committing to price caps on affected lines.

A more in depth inquiry into First Trenitalia will be conducted "should any proposals offered be considered insufficient", the CMA said.

A FirstGroup spokesman said: "We have been discussing our plans for the new West Coast Partnership franchise with the CMA for several months and we are pleased that on the vast majority of routes, it has found no competition issues.

"Now that this update gives more clarity, we look forward to submitting our proposals which we envisage will satisfy their concerns."

First Trenitalia is due to begin services on December 8.

Virgin Trains, a partnership between Virgin Group and Stagecoach, has run services on the West Coast Main Line since March 1997.

First Trenitalia has also been appointed as the "shadow operator" for HS2 and will be responsible for new high-speed services, depending on the outcome of an ongoing review.