LONDON -- American International Group Inc. has been forced to post more than GBP 500 million ($727 million) as collateral to cover possible defaults on rental payments on properties in Canary Wharf leased by Lehman Brothers Holdings Inc. and Citigroup Inc.

The requirement to pay collateral was triggered by a fall in the credit rating of AIG, which guaranteed payments on the leases, according to a securities filing by Songbird Estates PLC, the landlord for the properties.