The study published by the Journal of Medical Ethics said that many pensioners are being euathanised by their GPs even though they had never expressed a desire to die unnaturally.
It claimed that around one in 60 deaths that happened under GP care involved someone who had never requested euthanasia.
Research also showed more than half of the patients killed without giving their consent were over the age of 80, while two thirds of them were in hospital but not suffering from terminal illness.
"The Belgian population should be aware of the present situation and know that if their lives may come to the point where physicians think they are not worth living, in the absence of specific living wills advising physicians what to do then, they might be put to death," said report author Raphael Cohen-Almagor of Hull University.
Christian charity CARE said the report was an example of why assisted dying was a dangerous precedent.
It said the findings were a "serious warning of the consequences of following the same path as countries like Belgium".
CEO Nola Leach said: "I hope MPs pay close attention to this stark and very serious report, the contents of which is frightening and disturbing.
"It is clear a real culture of euthanasia has developed in Belgium in the 13 years since it was legalised.
"I think what this report shows very clearly is that no matter how many safeguards you attempt to put in the legislation, it will never be enough to protect vulnerable people in our society.
"The slippery slope argument sounds like a cliché until reports like this show it is not merely scaremongering, but a very real warning that if you open the door to assisted suicide, it will get pushed and pushed until more and more people are included in its remit.
"As a society we should be helping people to live, not to die and this report shows us why we need to think long and hard before so radically changing our whole approach to end of life care in our country."