LONDON (Bloomberg) — Prudential Plc, the UK insurer that's agreed to buy American International Group Inc.'s Asian unit, tumbled for the second day amid concern about the price of the deal and plans for the company's biggest ever rights offer.

Britain's largest insurer has fallen almost 20 percent in the past two days, after saying it will buy AIA Group Ltd. for $35.5 billion in cash and stock. The shares tumbled 42.5 pence, or eight percent, to 487.5 pence in London trading, valuing the company at £12.3 billion ($18.4 billion).

"People have just decided that if they are going to support it, they're selling existing shares to pay for the rights issue when it comes along," said fund manager Colin Morton.