The Labour Party has said it will take over the selection process for the Falkirk seat at Westminster.
The move to put its Falkirk constituency party under "special measures" came after allegations that some unions were packing membership lists to influence the decision.
The sitting MP, Eric Joyce, resigned from Labour after admitting assaulting four people in a House of Commons bar.
Labour is choosing its candidate from an all-woman shortlist.
A Labour Party spokesman said: "After an internal inquiry into the Falkirk constituency we have found there is sufficient evidence to raise concern about the legitimacy of members qualifying to participate in the selection of a Westminster candidate.
"As a result, NEC officers today decided a series of measures are needed to uphold the integrity of the Labour party.
"The Falkirk Westminster constituency is placed under special measures and the General Secretary will review internal membership procedures and advise on any changes that may be needed to ensure that they are not open to abuse."
Anyone who joined the Labour Party in Falkirk after 12 March 2012, when Mr Joyce announced he was stepping down, will not be allowed to take part in the selection process.
The Unite union has criticised the Labour Party's decision.
In a statement, it said: "Unite rejects the decisions taken today by the Labour Party in relation to the Falkirk West selection process. It does so on behalf of the many decent trade unionists who have joined the Party in good faith and are now to be denied any say in the choice of their Labour parliamentary candidate.
"None of the allegations contained in the report of the so-called 'investigation' have been put to Unite in clear breach of natural justice. The intervention by Party officials into this process has been driven by Blairite pressure to exclude trade unionists from any influence in the Party, an ambition clearly spelled out by Peter Mandelson last month.
"Trade unionists will draw their own conclusions regarding the integrity of the Party's procedures."