A method for determining a length of a message block of k bits from a length candidate set, comprising steps of: —Selecting (401) a length candidate (N) among the set; —Decoding (402) a received frame to form a decoded sequence that includes a message of a length equal to the length candidate (N), by a Viterbi decoder; —Calculating (402) a Viterbi variable (S(N)) for this candidate (N), —Comparing (403) the Viterbi variable with a threshold (Δ), —Repeating the selecting (401), decoding (402) and calculating (402) steps if said Viterbi variable (S(N)) is greater than the threshold (Δ) and if there exists an unselected length candidate among the set; —If the Viterbi variable is greater than a best value (Sbest), updating this best value to the Viterbi variable and updating a best length (Nend) to this candidate; —Wherein the threshold (Δ) is initially set to a value determined according to the conditions of reception of the receiver.