Two British museums have warned that international co-operation could be at risk after Aboriginal artefacts were seized while on a loan at an exhibition in Melbourne.

The Dja Dja Wurrung, an Aboriginal tribe originally inhabiting the Bendigo region of central Victoria, have secured an emergency declaration order preventing the return of the artefacts to Britain.

Fire-blackened bark etchings and a headdress, made by Dja Dja Wurrung ancestors 150 years ago, were loaned by the British Museum and the Royal Botanic Gardens at Kew to Museum Victoria in March.

"The emergency declaration puts at risk the very legal framework that allows such exhibitions to take place drawing on loans from Europe and America," said a joint press statement from the two British museums.

The Aboriginal bark etchings, thought to be the earliest in existence, depict images from the 1850s including kangaroos and hunting scenes.

They were due to return to London this week from the Etched on Bark exhibition.

The Times newspaper reported on Monday that the seizure of the artefacts sent a tremor through the international museum community because it would have such an impact on future loans to exhibitions.

"I can understand the motivation of people in Australia in trying to seize the items, but it adds to the difficulty of international lending generally," Maurice Davies, deputy director of the Museums Association, which represents Britain's 1500 public collections, told the newspaper.

"The world gets more litigious and more and more unacceptable things happen, such as physical threats to items because of terrorism," he said.

Under Australia's Aboriginal heritage laws, the emergency protection order can be extended indefinitely, but the 2500-strong Dja Dja Wurrung are calling for a permanent order, The Times said.

"This matter is being resolved by the Museum Victoria and the Australian state and federal authorities," said a statement from the two British museums.

In Australia, Gary Murray, secretary to the Dja Dja Wurrung Native Title Group, which secured the order, was unrepentant.

"They [the artefacts] belong to Australia," he told The Times. "If we had your crown jewels, you'd be knocking our doors down."

"If the British museums want to invoke legal rights, we've got pro bono lawyers at a major law firm who are prepared to represent us," he said. "We are not fearful of the legal process. We're in for the long haul."

He is calling on Australian Prime Minister John Howard to ask the British Government to return the barks and the hundreds of Aboriginal remains held by the Natural History Museum.

He is also vowing to write to the Queen, who is head of state of both countries, and to British Prime Minister Tony Blair.

The British Museum's Australian-born curator of the Pacific and Australian collections, Lissant Bolton, said the etchings were sent to the museum by John Hunter Kerr, a Scot, in the 19th century.

"He had good relations with the Aborigines," Bolton said.