The issue, worth USD 8.9 billion, will be offered at almost half price and is part of a far bigger effort to plug a vast hole of British pounds 12.8 billion in the balance sheet.
Barclays made the announcement as it reported first-half results which showed a quadrupling of net profit, but the scale and terms of the rights issue drove the price of the shares down by 5.50 per cent.
The plans are aimed at meeting demands made last month by the Bank of England's Prudential Regulation Authority (PRA), which supervises the banking sector.
Following a review, the watchdog had ordered Barclays in June to increase the amount of equity it holds against total assets, a measure called the leverage ratio.
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Barclays said that the moves, and separate measures to shrink parts of its business, should push its leverage ratio to above 3.0 per cent, the minimum required by the PRA, by June 2014.
"After careful consideration of the options, the board and I have determined that Barclays should respond quickly and decisively to meet this new target. We have developed a bold but balanced plan to do so.
"The plan is a combination of: a rights issue; prudent reduction of our leverage exposure; issuance of additional tier one securities; and the retention of earnings and other forms of capital accretion.
"We believe this represents the right combination to meet the PRA's leverage target. It also enables us to maintain our planned lending growth and broader support of our customers and clients."
The plans emerged as the scandal-hit bank admitted that it suffered a net loss of British pounds 168 million in the second quarter, or three months to the end of June, compared with a profit of British pounds 746 million a year earlier.