The obelisk will be a "mock-up" of the skin of the gallery building

The �25m Turner Contemporary is due to open in 2007 and will be visible from all over the Kent resort.

The landmark building, on the pier and in the sea, will house three floors of galleries, a restaurant and a shop.

The obelisk will test the materials to be used in the building against the elements to which they will be exposed.

The structure, which has been specially made by the shipbuilders Harland and Wolff, is being put into position by crane at low tide on Friday afternoon.

Andrew Mason, from the company Davis Langdon, which is overseeing the project, said it would be a "mock-up" of the skin of the gallery building.

The gallery on the seafront is due to open in 2007

"It will have the paint systems, protective systems on it, so that it will be there to see how the elements affect what is being looked at on the drawing board," he said.

Three companies are currently bidding for the contract to build the building, which will house works by JMW Turner and also feature a gallery for contemporary artists.

Exhibition spaces will rise from the sea bed, while the entrance, cafe and administration areas are built on the stone pier.

The future of the obelisk is uncertain, although the director of the Turner Contemporary, Victoria Pomeroy, insists it is not public art.

"As far as I'm concerned the obelisk is a fantastic structure... but I think we'll come up with some innovative ideas on how to use it post its testing," she said.