Britain's Prudential laid out the case for its $21 billion rights issue on Monday, finally allowing the insurer to push ahead with its planned acquisition of AIG's Asian insurance business.

The UK's largest insurer revised its $35.5 billion offer after an embarrassing and unprecedented last-minute delay by the Financial Services Authority nearly two weeks ago, to boost its capital position and earn approval for the deal.

It will sell new shares at 104 pence, a 39 percent discount to the theoretical ex-rights price (TERP), as Chief Executive Tidjane Thiam hopes to woo wary investors -- several of whom have expressed discontent -- and salvage credibility.

Prudential -- which will become the largest foreign insurer in Asia if its audacious purchase of American International Assurance (AIA) goes through -- said it would offer its owners 11 new shares for every two shares held.

The company also gave targets of new business profit revenue synergies of $800 million pre-tax and pre-tax cost synergies of $370 million during 2013 and remittances of at least $1 billion per year from the AIA group in 2011 and onwards.

Prudential, which has been facing the makings of a shareholder revolt this month, saw investors turn increasingly mutinous after the surprise delay.

The group needs 75 percent of shareholders to approve the deal, key to helping AIA's bailed-out U.S. parent AIG repay its taxpayer debt. A vote had been due on May 27, but that is now likely to be delayed.

(Writing by Douwe Miedema; Editing by Erica Billingham)