Two British museums have warned that international cooperation could be at risk after the earliest known Aboriginal bark etchings and a ceremonial headdress were seized while on a loan at an exhibition in Australia.

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.

Made by Dja Dja Wurrung ancestors 150 years ago, the fire-blackened etchings and the headdress were loaned in March to Melbourne's Museum Victoria by the British Museum and the Royal Botanic Gardens at Kew, west London.

"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.

Thought to be the earliest in existence, the Aboriginal bark etchings 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 that the seizure of the artefacts has sent a tremor through the international museum community, because it will 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 1,500 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 2,500-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." Mr Murray 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 due to write to Queen Elizabeth, who is head of state of both countries, and to British Prime Minister Tony Blair.