The embarrassing details, published by the Guardian newspaper, reveal that Campbell suggested changes to the draft Iraq dossier published by the British government on 22 September, six months before it launched a war on Iraq with the United States.
According to Campbell's letter, six of his proposed changes were acted upon, four others were not while the other was already under way.
Amongst changes made were the removal of the words that failed to strengthen the case for war.
He questioned why the draft report said Saddam's sons "may have" the authority to launch chemical weapons, instead of "have". But Campbell's request for the removal of the word "may" was turned down by the Joint Intelligence Committee (JIC).
He also wanted the report to say Iraq had secured uranium, but was told by the JIC that the phrase "sought to secure" would have to remain.
Meanwhile, in a passage dealing with Iraqi dual-use facilities Campbell successfully argued that the phrase "could be used" be replaced with "are capable of being used".
He also successfully proposed that the section detailing how long it might take for Iraq to develop nuclear weapons be more clearly explained, although the letter does not give details of what changes were made.
Campbell is an unelected press secretary to the prime-minister, not even employed by the British Civil Service. He has no responsibility to gather, change or suggest security ‘tips’ to the JIC and no background in intelligence gathering.
Blair’s press secretary, however, denied a BBC allegation he personally inserted a claim that former President Saddam Hussein could launch weapons of mass destruction within 45 minutes into the report.
But the letter clearly shows he suggested almost a dozen changes.
The leaked admissions are expected to form a crucial part of the Foreign Affairs Select Committee's final assessment, due next Monday.
Ministers will be judged by fellow politicians as to whether they deliberately misled parliament and exaggerated intelligence reports over Iraq's alleged and elusive weapons of mass destruction, the daily said.
Significantly, Campbell refutes allegations that he personally "sexed-up" the 50-page dossier by insisting it state that Iraq could launch weapons of mass destruction within 45 minutes.
"The chairman of the JIC has also confirmed and authorised me to say that it [the claim] reflected recent intelligence already in the JIC's classified assessment and that I played no part in the decision to include the intelligence in the dossier," Campbell wrote.
Asked to comment on the letter, a spokesman for Blair's office said simply: "We await the [foreign affairs select] committee's report on Monday."
Campbell, 46, is a former tabloid newspaper political editor known in British political circles as the "sultan of spin".