A soon-to-be-retired CalMac ferry has been redeployed to CalMac's busy Arran route after the long-awaited return of the main vessel was hit by a new problem.

MV Hebridean Isles will start sailing to Brodick on the island from Thursday.

The main Arran ferry MV Caledonian Isles had been due to return this week after eight months of repairs, but a new gearbox fault was detected on Sunday.

Ferry users can currently only travel from Arran to the mainland via Troon on a chartered catamaran, or on a small ferry to Claonaig on the Kintyre peninsula.

MV Hebridean Isles, launched 39 years ago, is due to be retired in November because of the amount of work required to extend its service life any further.

It will sail from Troon on Thursday and Friday because high winds are forecast, but will operate out of Ardrossan once the weather conditions ease.

The ship had been recently serving the Islay route, but MV Lord of the Isles will provide backfill. Sailings to Lochboisdale on South Uist will be cancelled as a result.

A CalMac spokeswoman said: "Cancelling any sailing is always a last resort and we apologise to the South Uist community for the disruption that this will cause."

The ferry operator said it would try to restore the Lochboisdale service as soon as possible.

MV Caledonian Isles has been out of action since February when annual maintenance inspections revealed serious corrosion.

All of its engines had to be removed while the damaged steelwork was replaced at the Cammell Laird shipyard in Merseyside.

It had just returned to Scotland and had completed berthing trials on Sunday when metal fragments were discovered in a filter in the port gearbox.

The ferry is now expected to be out of action for at least a week while investigation and repair work takes place.

Another ferry, MV Isle of Arran, had been operating from Ardrossan while Caledonian Isles was away, but it was booked in for annual maintenance from Wednesday which could not be delayed.

CalMac has been plagued by soaring maintenance bills and breakdowns in recent years with many of its ships well beyond their expected service life.

On Tuesday in the Scottish Parliament, Labour's Katy Clark said she had been contacted by people worried about hospital appointments and a farmer unable to get stock to market.

She asked the transport secretary if she agreed "the ferry crisis is becoming a national emergency".

Fiona Hyslop said she was concerned about the latest breakdown, and had impressed on CalMac the need for "resilience cover" for Arran.

The situation should begin to ease later this year when the long-delayed Glen Sannox enters service on the Arran route.

The first of four new large ferries being built in Turkey will also enter service next year, along with Glen Rosa, the sister ship of Glen Sannox, which is also earmarked for the Arran crossing.