A letter linking Michael Collins to the assassination of a British general six months after the Treaty was signed has been discovered in documents belonging to a London solicitor.

Found in the archives of JH MacDonnell, the lawyer who defended the two IRA men responsible for the killing, was a letter sent by one of the assassins before his execution to Oriel House, where Collins had his secret intelligence office, the Irish Independent reports.

"The letter is dated July 17, 1922 and the fact that this young man is writing to Oriel House is very significant. This was where Collins had his secret intelligence office at the time. The fact that he is writing to Collins' office rather than the anti-Treaty side, which others blamed for the killing, is crucial. It would indicate that he was more on Collins' side than those on the anti-Treaty side,” said Ian Whyte, of Whyte’s auctioneers.

"This is the closest thing to a smoking gun as you are going to get. It links the head of the IRA in London and one of the men who carried out the assassination, Reginald Dunne, to Collins. He wrote the letter just before he was hanged for the killing."

On June 22, 1922, Field-Marshall Henry Wilson was returning by taxi to his home at 15 Eaton Place in Knightsbridge after unveiling a war memorial at Liverpool Street Station when he was set upon by Reginald Dunne, 24, and Joseph O’Sullivan, 24.

The assassination, which would help spark the Civil War, occurred six months after the Treaty was signed between provisional and British governments.

Said Whyte: "Originally the IRA command in Dublin ordered the assassination in early 1921. But when the Treaty between the IRA and the British Government was delivered in June 1921 the order to kill Wilson was put on hold. However, in 1922 the order was authorized once again. For a long time people have argued over who ordered the killing. Some said that while Collins signed the Treaty he still wanted revenge on Wilson for his actions during the War of Independence, though that could never be proven. Wilson brought in the 'Black and Tans' and people say he ordered a 'war of terror' rather than a 'war on terror.’”

The letters was auctioned on Saturday, November 8, 2014, and fetched between $6,240 and $8,740. A tricolor flag flown over one of the buildings occupied by the Irish Volunteers during the 1916 Rising was also auctioned.

* Originally publlished in 2014.