Milligan, 42, who has occupied senior positions at Five since its launch as Channel 5 in 1996, is expected to take a role at Sky's sales operation Sky Media. He will be senior to its sales director, Mark Chippendale.
Rumours about Milligan joining Sky have flourished since Dawn Airey, his boss at Five, jumped ship to run Sky Networks in September 2002 as the managing director.
However, sources suggested that a clause in Airey's contract prevented her from poaching staff from Five for 14 months.
The top role at Sky Media, at a time when Sky is looking to grow its advertising revenue, is seemingly too good an offer for Milligan to refuse after seven years running the sales operation at Five.
Reports last year had linked him to a sales director job at the merged ITV if the broadcaster had been forced into operating two sales houses rather than one. However, this came to nothing when a single ITV sales house was given the green light by the Competition Commission.
Milligan is likely to face the challenge of beefing up Sky's commercial operation to bring in more revenues. In addition, Sky's role in the commercial TV market has become more interesting following Ofcom's decision to allow ITV's rival sales operations to merge.
In theory, Sky's sales team could merge with Channel 4, Five or Flextech's IDS operation to make it a larger player.
Milligan has been targeted by Sky after overseeing the steady growth of Five's share of TV advertising revenue to close to 8% on the back of increased audience share.
A former salesman at Thames TV, Milligan was also the head of sales at the satellite channel UK Gold when it launched. This launch experience helped him to land the Five position and he was promoted from the role of sales director to the deputy chief executive of the station in 2000 following the departure of the chief executive, David Elstein, and the promotion of Airey.
Some tipped Milligan for the top role at Five following Airey's departure, but the broadcaster turned to Flextech's Jane Lighting as its chief executive.
Sky last week announced half-year profits of £283m, up 84%. It now has 7.2m subscribers, on target to reach 8m by 2005.
Milligan was unavailable for comment as Campaign went to press. A Sky spokesman said: "No comment."
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the Forum here.