Image copyright AFP

The election Theresa May is seeking for 8 June would have a useful side-effect for her - by helping the Conservative Party in its attempts to put the lingering embarrassment of the 2015 general election expenses affair behind it.

In 2015, former Conservative Party candidate Mark Clarke (since dogged by allegations of bullying and sexually inappropriate behaviour, which he has always denied) organised a set of election campaign buses that toured marginal seats with activists on board.

But the running costs of those buses and associated hotel bills, which the Electoral Commission thinks should have appeared on the expenses returns of the individual Conservative candidates, were entered instead into the party's national expenses return.

This has allowed the legitimacy of more than 20 Conservative MPs to be questioned.

Opponents have pointed out that if the costs of the activists' "battle buses" had been entered on the local expenses returns, then the Conservative candidates would have breached the strict local spending limits.

Image copyright Getty Images Image caption The way the Conservatives' "battle bus" spending was recorded is being investigated

The affair has become so serious that 14 police forces have sent files to the Crown Prosecution Service, the CPS, to decide whether or not charges should be brought against the MPs or their agents for knowingly filing false election expenses.

The files with the CPS - from the Avon & Somerset, Cumbria, Derbyshire, Devon & Cornwall, Gloucestershire, Greater Manchester, Lincolnshire, Metropolitan, Northamptonshire, Nottinghamshire, Staffordshire, West Mercia, West Midlands and West Yorkshire forces - cover more than 14 seats.

Probably around 20 MPs and their agents are being considered for charges. A total of 40 people. Certainly at least 30. The agents may be slightly more at risk than the MPs themselves.

The CPS has until the end of May or early June, depending on the seat, to make a final decision about whether or not charges should be brought.

The MPs affected insist they have done nothing wrong and point to an email they all received from Mark Clarke when he was organising the buses in which he said: "Our costs are met by donations contributed and declared via CCHQ [Conservative Campaign Headquarters].

"We fund all the hotel and transport. This is an election expense and is accounted for out of central campaign spend."

They say this shows that their agents cannot have knowingly filed false expenses.

Thanet 'very different'

If the CPS eventually decides against bringing charges for the seats in these 14 police force areas, there is one seat in Kent - South Thanet - that is very different.

This was the seat in which the Conservative candidate Craig Mackinlay narrowly beat UKIP's then leader Nigel Farage.

Here the Conservative Party spent thousands of pounds on hotels - £15,000 on one hotel alone - and the Electoral Commission says at least some of that should have been on Mr Mackinlay's individual expenses return.

Mr Mackinlay has been interviewed by detectives under caution.

But although Kent Police has been in discussion with the CPS, no file has yet been sent for consideration of charges.

The force has confirmed that its investigation is still continuing after the election announcement

Mr Mackinlay insists he is innocent of any offence and says he will stand again at the upcoming election.

"I have not been charged with anything and I maintain that I have not done anything wrong," he said in the hours after Mrs May's announcement.

Image copyright PA Image caption Craig Mackinlay (right) narrowly defeated UKIP's then leader Nigel Farage in South Thanet

Even if the CPS decides not to bring charges in any of the cases, the 2015 election results in the seats affected would have always had a shadow hanging over them, as would the government's narrow majority.

Now the candidates can stand again in their seats and - if re-elected - they can claim to have won fair and square this time.

If any of their agents are charged in the meantime, while it may be embarrassing, it will not necessarily be terminal for the MPs' chances of being re-elected.

It will be for the voters in each of constituencies involved to decide how their MPs have handled their 2015 election expenses.