The claim, Sovereign said in the letter, was "greatly in excess of her entitlement".
Ms Boag is a long-standing friend of Ms Pullar who supported her during her battle with ACC, including attending a December meeting with ACC which has sparked investigations by the police and the Privacy Commissioner.
In the letter, Sovereign noted, it had been given a list of members of Ms Pullar's "claimed support/advisory team".
The list included Sir Selwyn Cushing, Mr Key, Dame Jenny and Dr Wayne Mapp.
Mr Key was at the time the Leader of the Opposition.
He has said he met Ms Pullar when he first entered politics - which was shortly after her accident - but had not had any contact with her since he became National Party leader.
Last night, he issued a statement saying: "I have not been involved in any 'claims support' or 'advisory team' for Bronwyn Pullar.
"The claim in the letter that I was part of such a team in 2007, or indeed any other time, is wrong."
Dr Mapp, a former minister who was an MP at the time, last night told Close Up he had met Sovereign to discuss the matter
"I simply facilitated some meetings ... they ultimately led to a settlement."
The Herald understands that settlement was in excess of $1 million.
Ms Pullar did not respond to the Herald yesterday.
The letter preceded those that Nick Smith wrote in support of Ms Pullar's ACC claim while he was ACC Minister and which led to his resignation from Cabinet last week.
Green Party co-leader Russel Norman last night said the Sovereign letter "raises the question of whether John Key did declare his full involvement in this case".
"It also raises the question of whether, in the conduct of his private investigation into Nick Smith's conflict of interest, he too had a conflict of interest.
"These are questions that only an independent inquiry can legitimately answer."
Ms Collins, the current ACC Minister, yesterday initiated defamation action against Labour MPs Trevor Mallard and Andrew Little and also Radio NZ over allegations about the leaking of an email sent by Ms Boag to Ms Collins concerning Ms Pullar's claim.
She demanded an apology from the pair, but Mr Mallard said Ms Collins' action was "an attempt to gag members of parliament, inside and outside of the House".
Meanwhile, it was reported that when Ms Pullar emailed Dr Smith's letter to ACC in support of her claim last year, she did so using software enabling her to track each time it was opened and who it was forwarded to without the knowledge of the email's recipients.
Internet security expert Peter Gutman, of Auckland University, said such "web bugs" were uncommon.
"Spammers use it on a massive scale, and beyond that it's used only by security geeks."
