The end to their often ugly verbal stand-off is likely to take place in London some time in December but the fight has the potential to become one of the biggest domestic showdowns in the sport's history.
Harrison won an Olympic gold in Sydney in October 2000 and at the time Williams was the best professional heavyweight in Britain, but he was then and he is still now, a relatively low-key character.
Even before Harrison had thrown a punch for pay as a professional he made ludicrous claims that he was the best heavyweight in Britain and he could win the domestic championship in his first three fights.
Williams and his management team at the time were understandably annoyed at Harrison's rant, but during the next three years it was the gold medallist and not the far more experienced and seasoned Williams who attracted most of the headlines.
After various fights between 2001 and 2004, Harrison accused Williams of acting like a cockroach and also claimed that his verbal nemesis was a coward.
Williams during this period lost a couple of fights that he should have won and by last summer it was obvious that Harrison, who at the time was unbeaten in 15 fights was a bigger star.
There was genuine interest and discussions took place about the pair meeting but all talks vanished when Williams found himself in the opposite corner from Mike Tyson in July 2004 in Louisville.
Against the odds and after an incredible four rounds, Williams knocked Tyson out and the balance of power on the British heavyweight scene shifted back in his favour and away from Harrison.
For the first time in his career, Williams responded spitefully to some of Harrison's comments during the days and weeks after his victory against Tyson.
At about the same time Harrison moved from Wembley to Las Vegas and claimed that he was finished with British boxing. However, boxing is the one sport where absolutely anything is possible. And it seems that Harrison, who has been at odds with the promoter Frank Warren for five years, and Williams, who has also fallen in and out of contracts with Warren during the same period, have all agreed terms for a fight that is as good as it can possibly get.
It is not yet known if any of the dozens of bizarre titles available to heavyweights will be on the line, but Williams against Harrison is the type of fight that needs nothing other than a referee and about 20 square feet of roped-off canvas.