THE pro-UK Better Together campaign is facing a possible criminal investigation after it emerged that records it submitted detailing spending during the referendum campaign were incomplete.

The Electoral Commission has revealed that it is in discussions with prosecutors over possible breaches in legislation, and that the watchdog may also take its own action against the organisation that could lead to civil fines.

It came as it was revealed that almost £7 million was spent by campaigners on both sides of the debate in a bid to influence the outcome of last September's vote and thousands of invoices detailing spending was published for the first time.

A statement from the Electoral Commission said: "Registered referendum campaigners were required to submit to the Commission invoices over £200. The submitted return of the registered campaigner Better Together is missing some information making the return incomplete.

"For such matters relating to the Scottish Independence Referendum the Commission liaises with the Crown Office and Procurator Fiscal Service (COPFS) in Scotland.

"We are consulting with the COPFS in order for them to consider whether they will open an investigation into this matter. Depending on the response from the COPFS the Commission will then consider whether to take any further action itself. Any potential breaches of the legislation taken forward by the Commission will be considered in line with our enforcement policy."

A Better Together source said that no investigation was under way and that further information had been provided to the watchdog. They added that where it had not been possible to provide invoices, for example for online train bookings, bank statements had been provided.

A Crown Office spokesman said: "We will consider any report when submitted, and continue to consult with our colleagues in the Electoral Commission as we do in the consideration of any potential offence related to the referendum."

Meanwhile, it emerged that of the 42 organisations and individuals who registered as campaigners, Better Together had the largest bill at just over £1.4 million, although the pro-UK organisation spent only £2,000 more than the pro-independence Yes Scotland campaign.

The SNP reported the highest spending of the political parties, investing almost £1.3m in its effort to secure independence. Labour spent £732,482 and the Tories £356,191 in a bid to secure a No vote.

The five organisations were the only ones to spend more than quarter of a million pounds during the campaign, which culminated in 55 per cent voting No.

A full breakdown of what the cash was spent on has also been revealed. It shows that advertising, market research and mail shots were significant costs to campaigners on both sides of the debate, with staff expenses and transport also proving to be significant expenses.

The SNP was left with a bill of almost £35,000 for four days of helicopter hire, including an extra £210 to land it in the grounds of Edinburgh's five star Prestonfield House hotel. The party also spent tens of thousands on Facebook advertising, including £3,000 highlighting the support of comedian Russell Brand. The Scottish Liberal Democrats paid £440 to a balloon artistry firm, based in Wales.

Driving services for the Margo Mobile, which toured housing estates promoting a pro-independence message in honour of the late Margo Macdonald, were worth thousands to a taxi firm, paid for by the 1001 campaign.

Amongst other campaigners, the Grand Orange Lodge of Scotland paid Glasgow City Council £10,000 for costs associated with an event it organised in support of a No vote and another £3,000 hiring toilets for a Helensburgh Parade.

National Collective, a self-anointed group of 'creatives for independence', spend almost £3,000 on yellow t-shirts, £700 on hoodies, more than £2,000 on a camera, and submitted receipts for several Apple iTunes purchases of £6.99, £25 and £54.99.

Generation Yes, which targeted young voters, spent £2,000 on 10 day tickets to T in the Park, another £1,600 on eight weekend tickets and £240 on Irn Bru flavoured sweets with 'vote yes' written on.