Barclays' offer period for Dutch bank ABN AMRO will run until October 4 and the British bank's bid will be declared unconditional if at least 80 per cent of ABN shares have been tendered, a filing showed.

Barclays is battling against a consortium of Royal Bank of Scotland  Belgium's Fortis  and Spain's Santander , to buy ABN in what would be the biggest ever bank takeover. The RBS-led consortium's offer is worth 71 billion euros ($97.3 billion) and 93 per cent in cash, while Barclays 65-billion-euros offer is about 38 per cent in cash.

ABN shareholders can tender shares under the Barclays offer from August 7 until October 4, Barclays said in a filing to U.S. regulator the Securities and Exchange Commission recently. Barclays will declare its bid unconditional if at least 80 per cent of ordinary ABN shares issued have been tendered or are otherwise held by Barclays, and certain other conditions are met, such as obtaining regulatory approvals, the filing said.