MSPs will be given evidence on the Assisted Suicide (Scotland) Bill by several groups later today.
The Faculty of Advocates and the Law Society of Scotland will warn that it is not clear enough.
The bill was introduced by the late MSP Margo MacDonald last year and is backed by several of her colleagues and some religious leaders.
It's a refined version of a previous assisted suicide bill which did not pass in the last Scottish Parliament session.
David Stephenson QC is expected to tell Holyrood's Health Committee that the current proposed change in the law is not sufficient to protect people who help others end their lives.
The Faculty of Advocates said in a statement: "If Parliament is to pass legislation to protect persons from what would otherwise be the legal consequences of assisting another person to commit suicide, the Faculty considers it is important that such legislation is clear, readily understood, and not just by lawyers, that key terms are well-defined and not open to a variety of interpretations, and that the penalties for breach of the requirements of the legislation are spelled out.
"Otherwise, persons wishing the protection of the legislation will be unclear as to whether their acts are protected and may render themselves liable to prosecution for serious crimes or subsequent review of their conduct in a civil court.
"The Faculty considers that the Bill as currently drafted may not achieve these essential goals."
Duncan McNeil, convener of the Health Committee, said in a statement: "MSPs have a duty to scrutinise the detail behind any proposed piece of legislation, but given that this bill adjudicates on life and death, it is imperative that we robustly and transparently scrutinise what is being proposed.
"Our committee has already received the views of over 800 people and will now start over two months of evidence, hearing from those with knowledge, expertise and interest in how the proposed bill would operate in practice.
"I want to make clear that all MSPs on the committee will consider the evidence in front of them, question robustly our witnesses and consider carefully the implications before we reach a committee view."
