Labour peer Lord Hunt has warned of the risk to vulnerable people from "unscrupulous relatives" if assisted suicide is legalised.
Proposals are being considered in Westminster and Scotland, with Baroness Meacher's Bill returning to the Lords for a second reading this month.
Writing in The Telegraph, Lord Hunt warned that assisted suicide "will always be open to bullying and exploitation".
"The process cannot be made safe because the ill will be exposed to fears of being a burden or to pressure from unscrupulous relatives," he said.
Lord Hunt, a former Labour spokesperson for health and social care, said he could understand why some people supported assisted suicide after witnessing the "difficult" demise of his mother following a stroke.
While the experience forced him to reflect on his beliefs around assisted suicide, he says it has not persuaded him to support the Meacher Bill.
"Over those many hours I realised that while I could certainly sympathise with the compassionate instincts of those who support assisted dying, it remains my sincere belief that we owe those same loved ones a further protection from feeling pressured into ending their lives because of a fear that they might be a burden and from those who might seek to gain from the accelerated death of a relative," he said.
Lord Hunt cited Oregon, in the US, as evidence of the "heavy weight of familial pressures" that can come into play after research by the state found that over half of all patients who opted for assisted suicide said they had done so for fear of being a burden.
But he warned of the additional risk of financial abuse by family members.
"Copious evidence of elder abuse related to assisted dying in other jurisdictions leads to only one conclusion: any assisted dying law would be fundamentally unsafe for the elderly in our society," he said.
"Particularly vulnerable to familial pressures, elderly members of our communities would be left exposed to the very real danger of their premature death to serve the financial self-interest of others."
He added, "I will be urging peers to reject a Bill that, however kindly meant, would expose vulnerable patients to either experiencing pressure from relatives to die before their time or to feeling that they are a burden and consequently requesting an assisted death."