The post is expected by competitors to bring a multi-million-pound long- term pay package, but Barclays made clear that Mr Harrison's earnings would be performance-related.
Mr Harrison's predecessor, the late David Band, in one recent year earned a total of pounds 1.4m, including bonuses, making him the highest-paid executive at Barclays at the time.
Martin Taylor, the chief executive of the Barclays group, declined to reveal the remuneration package and said: "Bill is not coming here for the money."
He added: "Effectively what Bill is paid will depend on BZW's performance, and it will also depend on the performance of Barclays.
"Quite a lot of the directors' bonuses are in stock. At the first opportunity he will join the [group] board."
Mr Taylor said the bank had already decided to offer Mr Harrison the job before the death in late March of Mr Band, who had planned to move from the "strenuous" executive role at BZW to the deputy chairmanship.
Mr Taylor added that he was amazed at the 60 or so names mentioned as candidates for BZW in various publications, and he had in fact only spoken to five people about the job. Mr Harrison was "absolutely the right person" to spearhead BZW's development of its global business.
Apart from his deal-making skills, Mr Harrison stood out among the senior executives at the family-controlled Flemings as a no-nonsense executive with a Birmingham accent and a different background from the traditional top-drawer City merchant banker.
But what was noticeable because of the contrast at Flemings may pass unremarked at BZW, which was founded only a decade ago and is a much more socially mixed organisation than Flemings.
In Mr Harrison's three years at Flemings the bank acted for ING in acquiring Barings and Dresdner in buying Kleinwort Benson and it also represented the Wellcome Trust during the takeover by Glaxo.
Before Flemings, Mr Harrison was head of European investment banking at Lehman Brothers.
He has also worked in the oil industry alongside Sir Alastair Morton, co-chairman of Eurotunnel. Both were at British National Oil Corporation in the 1970s.
John Manser, chief executive of Flemings, said: "Bill has always been an ambitious man which is a good thing in investment banking.
"I find it difficult to think ill of a person who wants to fulfill his ambitions. I am flattered to see that BZW could only find what they wanted at Flemings," he said.
Mr Harrison will be on "gardening leave" during his three months notice period, but said he would be available to help finish projects at Flemings and "tidy up" during that period.