
The Barclay brothers' bid, if successful, would give them 72.6% voting control of Hollinger International, which owns The Daily Telegraph.
The mailing of a formal offer a week-and-a-half after the two were revealed to be in talks came as Hollinger International filed a lawsuit earlier this week to block Lord Black's deal to sell the Telegraph newspapers to the brothers for a knockdown price of £260m.
The brothers have made their formal offer in the face of a spate of rival bidders announcing their intention to join the battle. Chief among these is David Montgomery, the former Mirror Group chief executive who is putting together a knockout £1.1bn bid with venture capitalists 3i.
The Daily Mail & General Trust, owners of the Daily Mail, has said it plans a £500m bid. Yesterday, DMGT's Associated Newspapers ended its distribution deal with the Barclays-owned The Business, which was bundled with The Mail on Sunday.
Associated denied the move was because of the mounting rivalry between the two media firms as they go head to head in a bid battle for the Telegraph Group.
Richard Desmond, owner of Express Newspapers, has also told executives he will bid.
The legal action taken by Hollinger is aimed at protecting all of the company's investors. Hollinger has also adopted a shareholder rights plan, or "poison pill", which will be triggered if control changes hands without board approval.
Shareholders were furious with Lord Black last week when the £260m deal was revealed. It was thought to be a very low price for the group, which also includes the Chicago Sun-Times and the Jerusalem Post.
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