US President Mr George W. Bush is to announce his decision on whether to allow federal funds for embryonic stem cell research in a national TV address.
He will reveal his intentions in a 10-minute address to be televised across the US at a 1 a.m.
He has wrestled for months over the funding issue and has called on scientific, ethical and religious advice.
White House spokesman Mr Ari Fleischer said: "The president wants to share the decision with the American people himself so they can see and hear why he came to the decision he came to. He wants to share this directly with the American people."
On Mr Bush's recent visit to Europe, Pope John Paul II personally urged him not to fund the research.
Opponents insist it is wrong to use human embryos for research purposes but advocates say great medical breakthroughs could be made.
Stem cells are essentially blank cells capable of developing into all of the body's organs but not into a complete individual. These cells form inside an embryo a few days after fertilisation but in order to remove the stem cell, the embryo must be destroyed.
Whatever the decision, two US senators have said they will introduce a bill which would ban the creation and destruction of embryos for research, including through cloning.
On the face of it such a bill could handicap stem cell research which relies on huge quantities of embryos to fuel its work.
PA