Scotland's transport agency could struggle to provide lifeline ferry services to islands within its budget in future years, public spending watchdogs have warned.

Transport Scotland spending on ferries rose by 115% between 2007/08 and 2016/17, going from £97m to £201m - with subsidies to operators doubling to £169m over the period.

While the agency has made "significant progress" as part of plans to improve services, Audit Scotland said its ten-year plan was focused on the Clyde and Hebrides.

It revealed Transport Scotland has "no Scotland-wide, long-term strategy."

Audit Scotland added the condition of about half of the harbours used by operators was unknown.

It said: "This means the full extent of Transport Scotland's future spending requirements on services and assets is not known.

"In the context of limited public finances, Transport Scotland will find it challenging to continue to provide ferry services that meet the needs of users within its allocated budget."

Scotland has an estimated 66 scheduled ferry routes linking up islands and the mainland, with Transport Scotland providing subsidies to almost half - 32 - of these services.

Fraser McKinlay, Audit Scotland's director of performance audit and best value, said: "Ferries provide vital links for Scotland's island communities and overall they are performing well.

"But it's critical that they provide value for money at a time when all public spending - on the islands and the mainland - is under pressure."

Transport minister Humza Yousaf said more than £1bn had been invested in ferry services since 2007, which had brought the introduction of new routes, the procurement of new vessels and the roll-out of cheaper fares.

He said: "All of our investment is subject to strict financial control and management, and we will continue to support our ferry networks and the communities that depend on them."