The British Museum's (BM) decision to retain seven mummified Maori heads that have been requested by the Museum of New Zealand Te Papa Tongarewa has "surprised and disappointed" the author of an independent report into the claim.

Freelance museum adviser Tristram Besterman said the heads should be returned despite not clearly fitting into the BM's policy on human remains because "it is unlikely that the policy was intended to exclude such human remains".

In a statement, the museum said it was retaining the heads because it was unclear whether or not a process of mortuary disposal had been interrupted, and that it was unclear that the importance of the remains to Te Papa outweighed the significance and importance of the remains as sources of information about human history. But the museum has agreed to return nine bone fragments also claimed by Te Papa.

A Te Papa spokeswoman said: "Although it has taken more than three years for the British Museum to agree to the return of the köiwi tangata, Te Papa acknowledges this is an important development, and appreciates the decision made by the museum trustees.

"Te Papa will now continue discussions about the Toi moko heads. As these may take a bit longer to return to New Zealand, our focus remains on a long-term goal of ensuring the ancestors are reunited with their living descendants, and then placed in a final resting place."

The museum's trustees, who were responsible for the decision, may have been swayed by a separate independent report by Phillip Endicott, an evolutionary biologist at the University of Oxford, which said "the seven preserved heads are all in an excellent condition and likely to provide outstanding quality of DNA".

The museum's own report, compiled by section head of Oceania, Lissant Bolton, questioned whether Te Papa was in the best position to research the remains.

Besterman said: "I am puzzled by the reasons given for retaining one part of the claim and returning another.

"The underlying rightness of the British Museum to have these human remains is at the heart of the issue, not whether they meet its institutional criterion of usefulness to science."

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BM deputy director Andrew Burnett comments on human remains, p17