Vale of Glamorgan MP Kanishka Narayan says he will now vote against the assisted dying bill.
Last November, the Labour MP backed legislation on the principle of allowing terminally ill people to end their lives.
MPs in the Commons are expected to debate at the third reading of Kim Leadbetter's bill on Friday - with the vote expected to be very close.
But Mr Narayan told the politicial news website Politico that he had changed his mind, because the proposed law could "result in an intolerably high ‘false positive’ rate of those coerced or misinformed into death."
In a statement on Thursday, he said: "I believe in the principle that each of us ought to have autonomy over our own body."
"That principle supports belief in assisted dying where freely chosen. It also requires of us a belief in the most rigorous standard of safeguards against misinformed, coerced death."
He added: "To all those who clearly deserve assisted dying for themselves, I am deeply sorry that a general law for everyone cannot support your particular case."
Mr Narayan, who held a town hall meeting in Barry on the issue last Sunday, said he would commit instead to campaigning for major investment into palliative care - or for "clarity in late-stage medical decision making to relieve pain" and "on a deeper future process for a better assisted dying law".
His neighbouring colleague at Westminster, Cardiff South and Penarth MP Stephen Doughty, voted against the bill at its first reading last November.
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