Philip Hammond has responded to a newspaper sting by The Sunday Times.
He told the The Andrew Marr Show: "I think the revelations are deeply damaging to the individuals concerned and their reputations.
"There is no way that retired officers influence the way in which military equipment is procured.
"I am satisfied that the system we have is completely robust.
"But there is an issue. Firstly over whether any rules have been broken. Clearly at least one, possibly more of the individuals named in The Sunday Times piece were still under the terms of the two year restriction that applies after they have left the service.
"Secondly we have to look at the level of access that we give retired officers to the MoD and to serving officers. "
More top news
A doctor has told the inquiry into a fatal crash that he had told the driver to remain on "non-driving duties" after a previous blackout.
Dealing with migrants has become "part of the day job" for truck drivers, a haulage firm has told ITV News.
The Hearing Clinic has been hit with a £220,000 fine for bombarding people with millions of nuisance calls.