Conservative MP Nus Ghani is doing around seven hours' work a month for Artemis Technologies as it helps to develop new ferries for Belfast.
Ms Ghani, MP for Wealden, East Sussex, was a transport minister from 9 January 2018 until 13 February this year.
In September last year, while in office, the MP wrote a letter to the then Business Minister Chris Skidmore in support of a bid for funding made by Artemis Technologies to UK Research and Innovation.
The bid was later successful, with £33m being awarded,
She had also visited Artemis Technologies' offices while on a regional tour in early 2019, but said she had not had "any other contact" with the company while a minister.
She took up her role with Artemis Technologies - leading a consortium headed by the company to develop zero-emissions ferries for Belfast - in September this year, according to Parliament's register of MPs' interests.
The rules for ex-ministers state Ms Ghani should not lobby the UK government until out of ministerial office for two years and "not draw on" any "privileged" information available when she was in office.