The Church in Wales is opposing the assisted dying bill, arguing it will endanger vulnerable people.
The bishops have raised concerns that the legislation could devalue human life and lead to unintended negative consequences.
In a joint statement, they stressed the need to protect society's most vulnerable individuals from the risks associated with this measure.
The bishops said: "This is an extremely difficult issue over which different people, including Christians, will have arrived at differing views with the best of intentions.
"Nonetheless, the protection of the most vulnerable people in our society from the risks inherent in this measure must be the paramount issue, and for that reason, we cannot in good conscience support the proposed legislation.
"There is abundant evidence from those jurisdictions where this measure has been enacted that good intentions can swiftly lead to bad and unintended outcomes, and to the devaluing of all human life."
The bishops' concern comes amid a renewed debate in Parliament over the controversial issue of assisted dying.
The proposed bill, which advocates for the right of terminally ill, mentally competent adults to control the timing and manner of their death, is set to be voted on next month.
This will be the first Commons vote on this issue in nearly a decade.
The current law bans assisted suicide in England, Wales, and Northern Ireland, with a maximum prison sentence of 14 years.
Pro-change campaigners argue that assisted dying gives individuals with terminal conditions the choice to control their death if they find their suffering unbearable.
However, opponents, including the Church in Wales and the campaign group Care Not Killing, highlight the potential risks of legalising assisted dying.
They fear it could place pressure on vulnerable individuals to end their lives due to feelings of being a burden on others.
The bishops of the Church in Wales have called for an extension of palliative care.
They argue that this would ensure no limits are placed on the compassion society shows.
They said: "Our Christian faith has always been rooted in the reality of pain and mortality, but also in the incalculable value of each human person, irrespective of their social standing, their access to resources, or their physical or mental ability.
"In that spirit, shown to us in the person of Jesus, we give our heartfelt support to the extension of the best possible palliative care to all who require it, so that no limits are put on the compassion which we show as individuals and as a society."
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