In a vote at its annual representative meeting, 63% refused a motion to go neutral on the issue.
Some members had wanted the organisation, which represents doctors across the UK, to take no position on assisted dying.
Last year, a bill before parliament which would have allowed a doctor to help a terminally ill person to die was defeated by a huge majority.
At the time, the BMA had voiced its oppostion to assisted dying.
Board members today decided the organisation would continue to oppose any change in the law by 198 voted to 115.
A vote on whether to even discuss the issue at the meeting was only narrowly approved.
Public policy charity CARE, a prominent campaign group opposed to assisted suicide welcomed the outcome of the debate.
CARE Chief Executive Nola Leach said: "I am delighted this attempt to force the BMA into a position of neutrality on assisted suicide has failed.
"This was a cynical effort to try and silence a respected opponent of assisted suicide and it is clear a move towards neutrality would have been a stepping stone towards full support for assisted suicide.
"Adopting a neutral position on this ethically charged issue would have been highly dangerous and it would also have represented a colossal failure of leadership."
Sarah Wootton, Chief Executive of Dignity in Dying said: "Unsurprisingly, the BMA has chosen to remain opposed to assisted dying. By making this decision, the BMA is not only wilfully ignoring the evidence but also the suffering of dying patients.
"In an attempt to stifle debate, opponents of assisted dying speaking during the debate accused members of the public who were handing out briefings outside of intimidation - a disgraceful and entirely false claim which highlights their dismissal of public opinion.
"The BMA's stance is completely out of touch with the general public, 82% of whom support assisted dying. Just 7% agree with the BMA's opposition and 84% believe its stance should change.
"The BMA has never surveyed its own members about their views on assisted dying. How can it claim to speak on behalf of 170,000 doctors across the UK without ever having asked them what they think?"