An inquiry into allegations of vote-rigging in Falkirk could hang over Labour for months, even running into to the party's annual conference, it has emerged.

Although police have said they will not launch a probe, the Information Commissioner's Office (ICO) is conducting an inquiry into whether data protection laws were broken, an offence that can carry a fine of £500,000.

The investigation is expected to last "months, not weeks", according to a spokesman for the ICO.

Labour is carrying out its own internal disciplinary procedures alongside the ICO inquiry.

The selection of Labour's 2015 General Election candidate in Falkirk has been shelved until the disciplinary process has run its course.

Party leader Ed Miliband said earlier this month that an internal Labour report had shown "people were being signed up as members of the Labour Party without their knowledge".

He appeared to successfully draw a temporary line under the row by taking on the Unite union – which is at the centre of the allegations and was backing Falkirk candidate Karie Murphy – and announcing a series of reforms designed to change Labour's relationship with the trade union movement.

However, his gamble, which has already led to claims Labour will lose millions in union funding, is expected to create tensions within the party.

Mr Miliband has attempted to kick those arguments down the line by emulating Tony Blair's Clause 4 moment and announcing he will hold a "special conference" on the issue early next year.

But senior party sources say they expect the topic of the reforms will be a major battleground at this year's annual conference in Brighton in September. Many within Labour already believe the week-long event will be crucial to Mr Miliband's campaign to win over the public.

The Labour leader silenced many of the naysayers within his own party with a powerful conference speech last year in which he made the surprise move of adopting the former Conservative slogan of "One Nation".

But in recent months Labour has suffered a slump in the polls and seen its lead over the Conservatives narrow.

A spokesman for the ICO said he could not say when its inquiry would conclude, but added that in general investigations by the office took months rather than weeks.

The probe began a few weeks ago, before the Conservative party referred the Falkirk allegations for investigation.

A source said a proactive decision had been made to look at the claims.