Westminster's standards watchdog has found that the DUP's Paul Girvan breached the MPs' code of conduct by writing to businesses in his constituency asking them to support a Christmas appeal he was launching.
However, standards commissioner Kathryn Stone concluded that the South Antrim MP's breach was "inadvertent" and "at the least serious end of the spectrum".
Mr Girvan has apologised for the breach and refunded the £149 he spent on stationary and postage costs.
The Westminster watchdog was alerted by a complaint from a businessman in the DUP MP's constituency who had six letters from Mr Girvan on House Of Commons notepaper posted through his letter box.
The letters referred to the "MP Christmas Appeal" and called on businesses to drop donations for food banks at a number of locations, including DUP offices.
Ms Stone said the matter was now concluded.
