Labour has suspended its selection process for a Westminster seat following concerns about "membership recruitment".
The move follows reports that the Unite trade union had recruited dozens of new members in Falkirk, potentially giving it the chance to influence the choice of parliamentary candidate.
The seat is held by ex-Labour MP Eric Joyce, who resigned from the party last year after a brawl in a Commons bar.
A Labour Party spokesman said: "We have suspended the start of the selection process of the Falkirk parliamentary seat.
"Concerns have been raised about membership recruitment which need to be investigated. An officer of the party will carry out an investigation to ensure the integrity of the process."
A source said the party was taking "swift and decisive action" now as the selection process was due to begin on Sunday.
On his blog earlier this week Mr Joyce said Stephen Deans was the chairman of both Falkirk West Constituency Labour Party and Unite in Scotland.
Mr Joyce wrote: "Apparently, Unite recruited 100 new members to Falkirk over a very short period."
He added: "I think it's great that so many people want to join the Labour Party. The UK party (i.e. the actual Labour Party) seems unsure about it all, though."
A Unite spokesman said the union was "fully confident that in relation to Falkirk West, as in all seats where our members are active in the Labour Party, our conduct is correct and fully compliant with the rules of the party".
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