JO Swinson, the deputy UK leader of the Liberal Democrats, is facing “serious questions” over her election expenses after thousands of pounds were left off her official declaration.

The East Dunbartonshire MP claimed her spending scraped in £210 below the legal limit in the party’s top Scottish target - but only after almost £7000 of costs were disregarded.

Despite the seat’s importance to the LibDems, the former minister said £2700 of leaflets and other material bought by the party went undelivered and so didn’t count toward her total.

Her official declaration said more than 93,000 items were never used, including around 26,000 copies of her main 4-page colour newsletter, the East Dunbartonshire Standard.

A further £4,040 of costs were omitted on the basis it was “national” spending promoting the LibDems in general, rather than Ms Swinson as the local candidate.

No other candidate in East Dunbartonshire split their spending on national and local lines.

It is an offence punishable by a year in prison to knowingly submit a false spending return.

An SNP spokesperson said: “Voters will find it hard to believe that so much cash could be spent on leaflets that just vanished. It was an election, not an illusion show.

“These revelations raise serious questions over the LibDems’ campaign in East Dunbartonshire. People have a right to know what has gone on here.

“The Lib Dems should know better after previous probes into their spending. There could be serious consequences if it is found that electoral rules have been flouted for political gain.”

The LibDems insisted all spending had been “apportioned correctly”.

Ms Swinson, 37, first won East Dunbartonshire in 2005, and was the MP for a decade before losing to the SNP's John Nicolson in 2015.

She was defeated by 2167 votes, the smallest margin of any Scottish LibDem loser, and the seat was her party’s number one target in June.

Ms Swinson regained it with a 5339 majority and later became UK deputy leader.

On her signed candidate return, which has been seen by The Herald, she declared she spent £14,409 out of the £14,619 maximum set to ensure a fair contest, around £4270 more than Mr Nicolson.

She said £2495 worth of material sent to voters was “national” spending as some space on her leaflets promoted the LibDems as a party and some promoted her.

Various leaflets were split 50:50, 75:25, 67:33 or 33:67 in respect of local and national costs.

The Electoral Commission allows split spending based on an “honest assessment”, but says the “main purpose” of the material is key.

It warns: “You should not split costs if an item is produced mainly to promote a candidate, and uses the party’s name or refers to the party’s policies purely in support of that aim.”

Ms Swinson also said 30 per cent of a £2922 bill for staff costs and a £2232 bill for office costs was national spending, taking another £1546 off the local total.

The Scottish LibDems, whose HQ is in Edinburgh, said some staff worked on its national campaign at Ms Swinson's office, a spartan unit on a business park in Milngavie.

However Electoral Commission guidance says “if the main purpose of activity in the office becomes promoting the candidate, the full costs of the office... should be reported as candidate spending”.

Ms Swinson said 70 per cent of the office expenditure was local.

Candidates are permitted to deduct the cost of unused materials, but must retain it "or evidence it was destroyed”.

The Scottish LibDems’ have attracted controversy before over their election spending.

On the eve of June’s poll, the Scottish LibDems' election manager, MSP Alex Cole-Hamilton, was reported by the police to the fiscal over alleged over-spending in his Edinburgh Western seat in 2016.

He had split his office costs down constituency, regional and local party lines.

After the 2015 general election, police also investigated several Tory MPs in England over claims local costs were wrongly described as national party expenditure to massage bills.

Craig Mackinlay, 50, MP for Thanet South, is now due to stand trial next May.

A LibDem spokesperson said: “All expenditure in this election was apportioned correctly and clearly identified in our election return which is now a matter of public record.

“It’s understandable that the SNP are smarting from a disappointing result, but they should accept that they were beaten in a fair fight in seats right across Scotland, and concentrate instead on the important business of running the country."