Since King Robert III gave the royal nod over 600 years ago, they have been taking people to either bank.

But now the last operational ferries on the upper reaches of the Clyde are in jeopardy, their futures thrown into doubt by high maintenance costs, increasing need for public subsidies and environmental impact.

The Renfrew to Yoker ferries, which have survived the death of the shipyards, the construction of the Clyde Tunnel and Kingston Bridge and the decommissioning of other services at Govan, Partick, Whiteinch, Finnieston and Erskine, will now be the subject of a detailed study to determine their viability.

Their owners, Strathclyde Partnership for Transport, will seek approval this week for part-funding for the £60,000 study, the balance being paid by Scottish Enterprise.

Consultants will be tasked with investigating other options for a crossing at the 200-metre stretch, arguably Scotland's shortest ferry crossing, with both agencies keen to see the costings for a no-frills pedestrian and cycle bridge.

The vessels, the Renfrew Rose and the Yoker Swan, will be beyond repair in less than two years, the cost of an upgrade almost as much as the estimated £2m for replacement craft.

However, of greater drain on SPT's resources is the annual £400,000 currently required to subsidise the ferries.

Some 150,000 passengers use the ferries every year at a cost of £1 per adult for a single crossing but every journey is subsidised by £2.65.

Originally operating half-a- mile upstream, the ferry moved to its present site 200 years ago to service rapid urban growth around Renfrew during the early industrial revolution. Throughout most of last century it serviced the many shipyards on both sides of the river and as recently as 1984 chain-operated ferries were still in operation.

But while other services were axed, difficulty in tunnelling or bridging where the ferries operate allowed them to survive.

Despite the dearth of publicity or information about its very existence, on a blustery Sunday afternoon there was a steady stream of passengers making the three-minute crossing from Glasgow to Renfrewshire yesterday.

A group of teenagers were making their way from Clydebank on the north side to the ski and cinema centre at Braehead on the southern shores, while several passengers were breaking the monotony of walking the dog with a short hop across the Clyde.

While the ferry has not transported cars since the 1980s, most crossings yesterday carried at least one cyclist.

The run-down surroundings of both slipways, the welcoming committee of swans and continual need for crew members to shovel grit when berthing suggest an era not in keeping with a gentrified Clydeside.

Steve Cairns has been using the ferry for 40 years. The 48-year-old barber from Erskine cycles every day to work and believes the combined weight of all agencies involved in the Clyde's regeneration should be pulled to save the service if the study says it should go.

He said: "They should be making a big deal about this. I just don't know why they don't use it as a river boat, servicing Braehead and heading right down to Finnieston."

Daniel Matursik, of Krakow but now living in Yoker, uses the ferry for work in a Renfrew bakery. He said: "A bridge would be good but I like the ferry because it is unusual."

An SPT spokesman could only confirm a report on the feasibility study would go before committee this week.