The island's population ebbs and flows like the tide around its shores, but now there are fears its precarious community is being undermined by the owner, the National Trust for Scotland.

Residents of Canna warn efforts to grow a sustainable number of inhabitants are being hindered by the charity's housing policy that affords no long-term tenure to those who want to make their home on the island, the westernmost of the Small Isles archipelago in the Inner Hebrides.

Geraldine MacKinnon, chairwoman and a director of the Isle of Canna Community Development Trust, has now written to Kate Mavor, the NTS's chief executive saying, the tenure situation appears worse than ever and fears people will leave again.

She says: "We find this situation unacceptable and unreasonable given the commitment people have made to give up everything and come and live here and the efforts being made to retain a viable community.

"This situation is something we feel would never be allowed to happen in any other small, fragile, remote community in Scotland without it raising major concerns and possible homelessness risks, especially in the case of a family with four children."

Currently, there are three households on a month-to-month lease following the expiry of short assured tenancy (Sat) agreements. One has four children representing the entire roll of the island's primary school.

There were 22 people on the island in 2010 but two years later only nine. Several who came to settle left citing the lack of secure housing. The population has recovered, standing at 21, but the fear is that number could reverse if something is not done to give more secured tenure to those who want to stay with their families.

Gordon and Denise Guthrie took their four children to the island in April from Motherwell. They say the NTS had even talked of a 25-year lease but they preferred two six-month leases.

Mr Guthrie is working as a web designer from their island home, but their lease is now from month to month. Mrs Guthrie says they are now worried other families will not be attracted to come to live on the island. "We were hoping for another family to come. Our children need friends. There is one other girl who is 17."

Stewart Connor went to Canna over four years ago as the NTS's property manager, moving from East Lothian where he lived with his partner Julie. But he says he resigned from the NTS last year after becoming disillusioned by senior management policy.

He said: "When I left the NTS we had to move from my tied house to rented accommodation. Our lease has run out so we are on a month-to-month lease. When I was in post we did a property tenure study in 2011 and at that time we were offering a minimum lease to new tenants of five years with notice of one year by NTS and the tenants two months. Now they won't offer anything other than a six-month tenancy and when that runs out it goes on to a month-to-month basis. "

He said a viable community couldn't be built on such foundations. Another real test would be NTS's response to residents' plans for a community hall. "We want to build it on a piece of land which has no conservation value whatsoever. It is covered by rabbit warrens and has the remains of an old coal shed on it. So we will want the NTS to gift us that piece of land."

Colin Irvine, 46, and his partner run the island's guest house and opened the former St Edwards Chapel as the Camus Arts Centre. He said: "This must be sorted out. The NTS don't want to go down the road of any asset transfer. But we can't build a community here under these circumstances."

He said the schoolteacher's lease says that if the NTS needs the house, she would have to move out in two months. "Where is she going to go?"

The NTS is already reviewing its role as a major landowner, and a spokesman said senior managers were on Canna last Monday for a meeting with the development trust, which had been constructive and helpful.

A new draft management plan was discussed and the joint partnership forum is to convene again as soon as possible where tenancies would be addressed.

But he stressed: "Current provision on Canna is no different from anywhere else. The length of tenancies available differs and the final choice is always down to individual tenants through negotiation.

"The trust is not in a position to discuss individual agreements."