Tory Brendan Clarke-Smith "outraged" locals - by sending out "thousands and thousands" of taxpayer-funded letters to pensioners boasting about Conservative policy - after the election was called.
It’s understood complaints have been made about the letters to the Police, parliament authorities and the Speaker, Lindsay Hoyle, about the letters sent to homes in Bassetlaw, the constituency he represented.
MPs standing for re-election are not allowed to describe themselves as MPs during election campaigns so they don’t have an unfair advantage over challengers.
And any communications with voters after the dissolution of Parliament would have to be counted against his election spending limit - around £20,700. Parliament was dissolved at 00:01 on the morning of Thursday 30 May - and at that point all members cease to be MPs, and become mere candidates.
But the letters under Mr Clarke-Smith’s name are postmarked 7.30pm on Thursday 30 May - some 19 hours after the House broke up. The letters were printed on Commons headed paper and franked with taxpayer-funded postage.
"This is almost certainly his entire election expenses," a Labour source said. "Thousands and thousands have gone out - every pensioner in the constituency seems to have got one. We've had about 100 complaints. People are outraged about it." They added: "He's attempting to steal an advantage in the election. It breaks every rule going. And the taxpayer paid for it."
One letter announces the re-opening of Doncaster Sheffield Airport, and reads: "As your Member of Parliament I am committed to ensuring that this reopening marks a new chapter of growth and prosperity for our region."
Another tells voters about Tory Chancellor Jeremy Hunt's recommitment to the triple lock on pensions. And it mentions the "triple lock plus" policy - which increases tax thresholds to take state pension recipients out of taxation - weeks before it was announced by the PM during the campaign.
"I am actively lobbying the Chancellor to further support our pensioners by increasing the tax threshold so that no-one pays tax on their state pension," the letter reads. "I will continue to advocate for measures that enhance the financial well-being of our retirees and ensure that our policies reflect the respect and support they have earned."
Both letters are signed "Brendan Clarke-Smith MP".
It comes after fellow Tory minister Johnny Mercer was accused of breaking rules by sending letters to people in neighbouring constituencies.
The Veterans’ minister sent letters, on Commons headed notepaper, to people living in parts of Plymouth that will come under his current seat after boundaries are re-drawn for July’s General Elections. But Mr Mercer won’t be the MP there by the time their homes are part of the constituency - he’ll be a candidate seeking re-election.
Commons conventions have long banned MPs from writing to constituents of other MPs.
Mr Mercer insisted he hadn’t broken any rules.