A row over the selection of a Labour candidate for the next general election took a fresh turn tonight when the union at the centre of the dispute accused the party of a "breach of natural justice."

Unite said Labour was "rushing ahead" with the process of choosing a parliamentary candidate in Falkirk after claims that the union was trying to "stitch up" the selection by cramming the constituency with new members.

Unite said it has written to the general secretary of the Labour Party demanding that the process is halted immediately pending a full discussion at the party's national executive committee.

The vacancy emerged when MP Eric Joyce was kicked out of the party after committing an assault in a House of Commons bar.

A Unite spokesman said: "The decision to rush ahead with the process of selecting a Labour parliamentary candidate in Falkirk, so denying a vote to a mass of members, and an imposed shortlist of candidates is without any justification and is a further breach of democratic procedures and natural justice.

"While the Labour Party has continued to deny Unite a copy of the investigation report into Falkirk CLP, yesterday officials from the union were provided the opportunity to study the report at the party's head office.

"As a result, Unite is more than ever convinced that the measures that have been taken on the basis of the report, in particular the disenfranchisement of around 150 Party members, and the imposition of a regime of "special measures" on the CLP, are unnecessary and are at best an extreme over-reaction, at worst the product of an anti-union agenda.

"In particular, Unite notes that the allegations regarding membership malpractice affect no more than a handful out of those who have now been excluded from the selection process.

"Even if the allegations of irregularities are proved to be correct in that small number of cases - and that remains highly questionable - then this mass exclusion is utterly disproportionate.

"Unite remains confident that its activities within Falkirk were entirely within the rules of the Party as they stand. It has done what the Labour Party says it wants - recruit ordinary working men and women to the party.

"It is clear that the decisions taken by the Labour NEC officers last week on the basis of a selective and sensational reading of small extracts from the report need to be reviewed as a matter of urgency, and that no action should be taken on the basis of those decisions without a full prior debate at the NEC itself."

Labour said last week: "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.

"The General Secretary will review internal membership procedures and advise on any changes which may be needed to ensure that they are not open to abuse.

"The freeze date for the Falkirk Westminster parliamentary selection will be set at 12 March 2012."

A senior Labour party source said: "We have acted swiftly on this matter to uphold the inegrity of the Labour Party."