Oliver Pauley, UK managing director, told PRWeek this morning: "It’s completely untrue and it’s a ridiculous conspiracy theory. I don’t think there’s anything more to say on it than that it's complete nonsense."
The agency was catapulted on to the national news agenda yesterday, after Len McCluskey, general secretary of Unite, accused it of orchestrating a coup against Corbyn.
Speaking on The Andrew Marr Show on BBC1, McCluskey blamed "sinister forces" for the plight of the embattled Labour leader.
Challenged to explain what he meant, he responded: "Well, if the BBC want to do an investigatory programme of a company called Portland then feel free to do so. This is a PR company with strong links to Tony Blair, right-wing MPs, who have been critically involved in this orchestrated coup."
Portland was founded by former Blair aide Tim Allan and lists Alastair Campbell, Blair’s former PR man, as one of its senior advisers.
The claims against the agency come amid mounting speculation over Corbyn’s future as Labour leader.
He was humiliated last week when 172 Labour MPs backed a motion of no confidence in his leadership and most of his front bench team resigned.
When asked by PRWeek whether any individuals at Portland had given informal advice to Corbyn’s opponents, Pauley admitted: "The answer is I don’t know, I have no idea at all but the accusation that the agency has been in any way involved in this is complete nonsense."
To date, the only formal response by Portland has consisted of a single tweet yesterday:
Len McCluskey's comments on #Marr. This is a ridiculous conspiracy theory and completely untrue. He should withdraw.
However, the agency was aware of the allegations before the remarks were reported in the mainstream media.
Claims over the involvement of Portland in the coup against Corbyn have been made repeatedly by a left-wing news website, The Canary.
Last week saw staff at Portland aggressively door-stepped by an unnamed journalist. A video posted online shows two women being subjected to a six-minute tirade by the journalist, who only left the building after security was called. The reporter, who secretly filmed the whole exchange, is heard muttering "the consequences will be worse for you" after being denied an interview or appointment to see anyone at Portland.