The assisted dying bill may fail to become law if it progresses its current rate, the peer steering it through the Lords has admitted.
In a letter to his fellow peers on Wednesday evening, Lord Falconer said the upper house may fail to complete the scrutiny process in time.
If the bill is not passed by the Lords by the end of the parliamentary session, expected in May, then it will fall.
Lord Falconer said: "We must be clear with ourselves. If we continue at the rate we are going, this house will fail to complete the process of scrutiny.
"We will reach no conclusions on the bill as to how it should be amended or whether it should return to the Commons."
MPs voted to approve the bill - a private members bill put forward by Labour MP Kim Leadbeater - in June 2025.
It then moved to the Lords to be scrutinised by peers - where it has made slow progress.
Lord Falconer separately said this evening that if this bill fails, "the Parliament Act is an option".
Under the Parliament Act, legislation can be passed without the approval of the Lords if the Commons has backed it twice.
But it is a rarely used power - and it has never been used to force a private members bill through.
Supporters of the bill say they would attempt to reintroduce the bill after the Kings Speech, expected in May, if the first bid fails.
The Terminally Ill Adults (End of Life) Bill was approved by 314 votes to 291 at its third reading in the House of Commons - a majority of 23.
If passed, the bill would allow terminally ill adults with fewer than six months to live to apply for an assisted death, subject to approval by two doctors and a panel featuring a social worker, senior legal figure and psychiatrist.