The National Museum of Scotland is coming under pressure from Westminster MPs to return one of the Shetland Islands' greatest treasures from Edinburgh to the newly opened museum in Lerwick.

Alistair Carmichael, the MP for Orkney and Shetland, is leading the fight to have the Saint Ninian's Isle treasure returned to its rightful home in the islands, and already he has won cross-party support. While the National Museums of Scotland is accountable to the Scottish Executive, Angus Robertson, the SNP's Westminster leader, has thrown his weight behind the islanders' campaign.

Speaking to The Herald last night, Mr Carmichael said: "The St Ninian Isle treasure is virtually lost in the National Museum of Scotland. The Shetland Museum could display it to its full effect in a place where its significance would be truly appreciated. There is no good reason for it now to be left in Edinburgh."

The treasure of silver bowls and ornaments, which is believed to date from around 800AD, was found in 1958 by a schoolboy helping at an archaeological dig on the island, and ever since its discovery it has been controversial. Arguments abounded over its origin: whether it was a collection of ecclesiastical silver from some pre-Norse Celtic monastery or the treasure of some secular household.

Mr Carmichael has stepped up the islanders' campaign to retrieve the treasure because the new Shetland Museum and Archives, opened last Thursday by Prince Charles, the Duke of Rothesay, and Queen Sonja of Norway, has the facility to accommodate such an important part of the islands' heritage.

Not only does Mr Carmichael believe it is the appropriate place to house the treasure, he argued that it would boost the islands' growing tourist trade.

The St Ninian's Isle treasure is presently housed in the National Museum of Scotland in Chamber Street in Edinburgh.

Last night Dr Gordon Rintoul, director of National Museums Scotland, said: "National Museums Scotland care for the national collections on behalf of the people of Scotland. We aim to make them accessible to as many people as possible. We lend significant numbers of items across Scotland and internationally, and we consider requests for loans on a case-by-case basis."

The trustees, chaired by Sir Angus Grossart, are responsible for setting the museum's vision, strategy and policy and for monitoring progress to achieve these but they are accountable to Scottish ministers and the Scottish Parliament.