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 percent 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 percent in cash, while Barclays 65-billion-euros offer is about 38 percent 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 late on Friday.

Barclays will declare its bid unconditional if at least 80 percent 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.

The RBS consortium's offer period started July 23 und runs until October 5, the group said last month. It also set an 80 percent threshold to declare its bid unconditional.

The filing confirmed Barclays' offer, which entitles ABN shareholders to receive 13.15 euros in cash and 2.13 new Barclays shares for each ABN share tendered.

Barclays is expected to formally launch its offer for ABN by a deadline to do so on Monday.