Standards committee press secretary Fiona Dick said: 'We have a duty to examine current concerns.'
Launching next month, the inquiry will gather evidence from, 'any interested parties', Dick said. This will include ministers, advisers, civil servants and commercial lobbyists.
Dick added: 'We will not be looking at individual cases - we are looking at the relationship overall.'
The inquiry is expected to last nine months, after which recommendations will be made to the Government.
Last week the extent of the problems within the DTLR's PR function was exposed. Both Moore and Sixsmith were forced to resign after the leaking of an e-mail accusing Moore of attempting once again to bury bad news.