There are "insufficient grounds" to launch a criminal investigation into alleged irregularities in the contest to select the new Labour candidate for the Falkirk seat at Westminster, police have said.

The party had handed over documents to police in Scotland amid claims that Unite - Labour's biggest trade union backer - had sought to swing the contest by cramming the constituency with new members.

But police said today there was not enough evidence currently to support a criminal investigation.

A Police Scotland spokeswoman said if additional information came to light, this would be examined.

She said: "Following a comprehensive review of all material submitted, Police Scotland has concluded there are insufficient grounds to support a criminal investigation at this time.

"However, should further information come to light, this will be looked into."

The row over the selection of a candidate to contest the seat, currently held by Eric Joyce, has sparked the biggest crisis for Labour since Ed Miliband took over as leader of the party.

The vacancy emerged when Mr Joyce was kicked out of the Labour Party after committing an assault in a House of Commons bar.

Last month Labour put the Falkirk constituency party under ''special measures'' after an internal report raised concerns over the selection of a candidate for the next general election.

This suggested that the local constituency may have been packed with new members in a bid to influence the choice of candidate.

Labour went on to hand over documents to the police about claims that the Unite union had tried to fix the result of the candidate selection process by packing the constituency with members whose subscriptions it had paid.

But the party's actions sparked a bitter dispute between it and Unite, whose leader Len McCluskey has dismissed the allegations as "nonsense" and has claimed there is a "smear campaign" against the union.

The dispute resulted in Karie Murphy, who was said to have been Unite's preferred candidate to fight the seat in 2015, and constituency chairman Stephen Deans being suspended from the party.

Meanwhile, election co-ordinator Tom Watson - who has close links with Unite - stepped down from the Shadow Cabinet.

In the wake of the Falkirk row, Mr Miliband has said he wants to "build a new way of doing politics'' and turn Labour into a mass-membership party ''powered by the people''.

Earlier this week, the Labour leader unveiled plans to hold a special conference next spring to seek the backing of his party for ''historic'' reform to its links with the trade unions.

Mr Miliband has already announced that he wants members of trade unions affiliated to Labour to be given the chance to ''opt in'' to membership of the party, replacing a system where their political fund levy is automatically passed on to Labour unless they opt out.

The move is designed to boost Labour membership from its current 200,000 and loosen the financial hold of union bosses over the party, but unions have warned that it could cost millions of pounds in lost donations.

A Unite spokesman said: "Unite welcomes the police decision not to investigate the Falkirk selection, which appears to be based on an overdue application of common sense to the situation.

"Unite reaffirms what it has always said - the union broke neither Labour Party rules nor the law in Falkirk. Those in the media who have smeared the union without evidence or justification should now hang their heads in shame.

"We would hope that Labour will now lift the suspensions of Stevie Deans and Karie Murphy, agree to an independent investigation into what happened in Falkirk, and restore full rights to the constituency party as soon as possible."

A Labour Party spokesman said: "As a result of the police decision, we will now pursue disciplinary action as a matter of urgency.

"The internal Labour inquiry found there was enough evidence to concern us about membership recruitment in Falkirk.

"We will act on this swiftly and thoroughly as we have done throughout this matter."

The Scottish National Party's MSP for Falkirk West, Michael Matheson, said: "It is clear that this complaint to the police was little more than a stunt to divert attention from the Labour Party's own failure to deal with the crisis in Falkirk, and a waste of valuable police time.

"Ed Miliband was happy to share this report with the police, but Labour's Scottish Executive has been kept completely in the dark - clearly London remains firmly in control.

"Johann Lamont claimed she was the leader of all parts of the Labour Party in Scotland, but it is clear that her leadership is in name only, and Westminster is still calling the shots.

"The people of Falkirk deserve so much better than this."