A bid was expected sometime this week, perhaps even today. BAe is thought to have decided to fire the opening shot with an offer worth around pounds 18 a share, valuing the Trident submarine maker at about pounds 700m.
Its rival, GEC, the defence and electronics giant, was expected to follow soon after with its own bid.
The rival bidders were forced to abandon their plans when the sale was referred to the Monopolies and Mergers Commission last year. Last week, in a controversial decision, the Department of Trade and Industry cleared the way for a resumption of the battle.
Neither company wanted to be the first to restart the bidding, although BAe has always said it would bid if it got the go-ahead from the MMC. Analysts expect the bidding to go to as high as pounds 20 a share.