The First Minister has told assembly members he cannot announce the decision on the proposed new motorway south of Newport until after the Newport West by-election has taken place.
Mark Drakeford said that it has been made clear to him that civil service rules on "purdah", an election period when major announcements are banned, affect the Welsh Government - even though it's a Westminster by-election.
He was answering a question from the Conservative leader, Paul Davies, who accused Mark Darkeford of making "one excuse after another".
Mr Davies said the First Minister should release the recommendations of a public inquiry into the project to reduce congestion on the M4 through Newport - believed to cost £1.4bn.
Mark Drakeford said last week that he had started reading the report of the inquiry inspector, which is over 500 pages long.
Eleven candidates are contesting the Newport West by-election, which was caused by the death of the veteran Labour MP Paul Flynn.