Parliamentary Standards Commissioner Kathryn Stone was looking at possible breaches to the MPs’ code of conduct covering earnings, gifts and foreign travel. Labour’s shadow foreign secretary apologised for the error and said he had put “revised systems in place” to prevent future late declarations.
This was included in an update to Parliament’s website after Ms Stone opened the investigation. The MP for Tottenham joins leader Sir Keir Starmer in being probed by the watchdog.
Sir Keir said he was “absolutely confident” he had not broken the rules, as he blamed “administrative errors” in his office.
MPs must register changes to their financial interests within 28 days.
A spokesman for Mr Lammy said he took his declaration responsibilities seriously.