Francie Molloy has been selected as the Sinn Féin candidate to replace Martin McGuinness as MP for Mid Ulster.
Mr Molloy, who is currently deputy speaker of the Assembly, was due to take over the role of speaker from Willie Hay sometime next year as part of a rotation arrangement with the DUP.
However, Mr Molloy is likely to resign from Stormont if he is elected to Westminster, even though he would not take his seat there under the Sinn Féin policy of abstentionism.
Another Sinn Féin member could then be co-opted in his place.
Mr Molloy is a veteran political campaigner who cut his teeth in the 1960s civil rights movement.
A prominent activist during the H Block prison protests, he acted as director of operations for Bobby Sands, the hunger striker, when he won the Fermanagh and South Tyrone seat in 1981. After Sands' death Mr Molloy fulfilled the same role for Owen Carron, who also won the seat.
He was on the Sinn Féin talks delegation in 1997 and was an unsuccessful European candidate in 1994. He has been a member of the Assembly since 1998.
There is a nationalist majority in Mid Ulster and this strongly favours Mr Molloy's re-election in a straight fight.
Mr McGuinness welcomed his selection. He said: "It is a great honour for me to propose Francie Molloy and for Michelle O'Neill MLA to second him as candidate to contest the forthcoming Mid Ulster by-election.
"Francie Molloy has been active in political and community life since his teens.
"He was one of the first people in his area to join the Civil Rights Movement and has been involved in local and national political life ever since, having first been elected in 1985.
"Francie will make an exceptional MP and representative for the people of Mid Ulster."
Mr Molloy said: "I am delighted to represent the party in the byelection in Mid Ulster."