CIVIL war has broken out within Labour after the party announced it was to impose Falkirk's next General Election candidate from Westminster in a move that has barred many local activists from taking part in the process.

Trade union Unite – the party's biggest financial backer – has seen dozens of its members frozen out from selecting their preferred potential MP, and has angrily accusing the party leadership of succumbing to "Blairite pressure" following the move.

A new candidate is required to replace Eric Joyce MP, who resigned from Labour after a brawl in a House of Commons bar.

Unite's response followed an announcement by the party's National Executive Committee (NEC) that it was putting Falkirk Constituency Labour Party on "special measures" .

This came after it considered an internal report into allegations the union had recruited dozens of new members to the CLP to help secure the candidacy of one-time frontrunner Carie Murphy.

Ms Murphy is a senior figure within Unite and works for MP Tom Watson, the Labour vice-chairman. Last month as the row took hold and the selection process was suspended, senior Labour sources revealed she had pulled out of the Falkirk race. The union has denied any wrongdoing.

The internal report found there was "sufficient evidence to raise concerns about the legitimacy of members qualifying to participate in the selection of a Westminster candidate".

Yesterday's meeting of the NEC – which party leader Ed Miliband did not attend – decided to introduce a "freeze date" for membership eligibility of March 12, 2012, when Eric Joyce, the local MP, resigned. This means dozens of trade unionists who have recently become members of Falkirk CLP will now be disbarred from taking part in the selection process.

Some estimates have put the number of new union members to Falkirk at "well over 100" but a senior Labour source declined to discuss figures. The NEC officers also announced Labour's general secretary, Iain McNicol, will conduct a review of internal membership procedures "to ensure they are not open to abuse".

Last night, Unite said it rejected the NEC's decision "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".

It argued that none of the allegations contained in the internal report "of the so-called 'investigation' have been put to Unite in clear breach of natural justice".

The union added: "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."

In May, Lord Mandelson complained that too many of Labour's selection processes were "in the hands of one union at worst or a couple at best".

Trade union donations account for around four-fifths of all the money Labour receives and Unite is said to be sponsoring around 25% of parliamentary candidates selected thus far for the 2015 General Election.

Len McCluskey, the Unite general secretary, has insisted he is simply trying to ensure the Parliamentary Labour Party is more working class.

In April, he clashed with Mr Miliband after warning him not to heed advice from Blairite figures in the Shadow Cabinet. He said: "If he gets seduced by the Jim Murphys and Douglas Alexanders, then the truth is he'll be defeated and he'll be cast into the dustbin of history."

The Labour leader was incensed and accused Mr McCluskey of disloyalty with his spokesman saying: "This attempt to divide the party is reprehensible."

The senior Labour source confirmed the Falkirk process would be an all-women shortlist, pointing out how only 27% of Scottish MPs were female. No time-frame has been set for a candidate to be chosen.