They were bequeathed to Croydon by local businessman and collector Raymond Riesco in 1959 and include Tang dynasty tomb models and Ming dynasty bowls dating from neolithic times to the 19th century.
The council are hoping to raise £13m towards redeveloping their Fairfield Halls property, www.museumsassociation.org reports.
Nick Merriman, convenor of the MA's ethics committee, said: "We do not have much information at the moment, but we understand they are proposing to sell these works to fund work on Fairfield. The council should follow the formal procedure according to the code of ethics. At the moment it is not clear to us that they are doing so.
"We would particularly like to know why the collection is not considered core, as we understand it was part of the founding deposit at the museum."
Terms of Bequest
A comment under the museumsassociation.org report from Judith Martin, project organiser of the Industrial Buildings Preservation Trust, said she had found a mention on the council's own website saying Riesco had gifted Oriental china "...to the Corporation on the condition that they were not split up".
She added: "If Croydon can't afford to show or insure the bequest, it should give the entire lot to the British Museum, where I gather some has been on loan anyway. Flogging bequests... is a guaranteed way to ensure no one will leave anything to the public estate again, thus impoverishing future generations even more."
A council spokesman toldATG: "The decision to sell the items, which are currently being stored in a secure central London location, comes after escalating insurance and security costs meant that the full collection was becoming too expensive to maintain. The council is currently seeking stakeholders' views including national organisations and museums.
"The proposal has the backing of the Riesco family, providing the proceeds of the sale are used to support culture in the borough. It would see income from the sale be invested in the redevelopment of the Fairfield Halls.
"There is also the possibility the Ashcroft Theatre could be renamed the Riesco Theatre in tribute to the family's contribution to the borough's culture."
The proposals are due to be considered at the council's corporate services committee on Wednesday, July 24.