The OSCR has ordered the charity, which manages the Scottish adoption contact register and provides services for people who have been adopted, to change its charitable objects.
It ruled that amendments made by the charity to its objects in November 2009, which the OSCR did not authorise, meant that its articles of association no longer displayed clear charitable purposes.
The charity's constitution says it is responsible for administering the Adoption Contact Register for Scotland and the Adoption Registry for Scotland.
The OSCR says these are subject to the direction of the Scottish government, and that administering them breaches the rules in the Charities and Trustee Investment (Scotland) Act 2005 because there is scope for ministers to control the charity's activities.
The report also says seven other listed charitable purposes, which include providing counselling to adopted adults and providing research and training facilities for people working in after-adoption services, do not meet the legal requirements for charitable purposes.
"It is not possible clearly and unambiguously to infer any of the charitable purposes in section 7(2) of the act from the wording of the charitable objects in clause 3(b)(i)-(x) of the charity's memorandum of association," the OSCR report says.
The charity's memorandum of association also allows it to transfer property to other organisations with similar charitable objects, if it is wound up. In the report, OSCR says it would be illegal for Birthlink to transfer property to organisations with objects that were similar if those objects were not charitable.
The report says the charity has failed the charity test under Scottish law. It says Birthlink must amend its constitution to make sure its purposes are exclusively charitable by 6 January, or it will be struck off the charity register.
Linda Powell, executive officer at Birthlink, said she was disappointed with the OSCR report, but said the charity would redraft its objects. She added that the charity was considering an appeal against the OSCR order.