Barclays has secured a cash injection from one of Japan's biggest financial groups as part of the bank's plan to raise more than £4 billion to bolster its balance sheet.

It is in the final stages of negotiating an investment by Sumitomo Mitsui Banking Corporation (SMBC) thought to be worth about 100 billion yen (£480 million).

Details of the deal with Sumitomo is expected to be announced next week alongside the names of at least three other foreign investors.

China Development Bank and Temasek, which already have stakes in Barclays, are thought to be planning to take part in the £4 billion placement, along with the Qatar Investment Authority.

The capital injection, underwritten by the new investors, is expected to take the form of a private placement of shares that would leave Japan's third-largest financial group with a stake of 2 to 3 per cent in Barclays.

Shares in the placement are likely to be issued at a small discount, with a clawback offered to existing investors. A prospectus will be issued with the placement detailing recent trading at Barclays, with the capital-raising likely to be completed by mid-July.

The tie-up between Barclays and SMBC is expected to result in the two banks deepening their alliance in Asia. The deal may include strengthened ties between the asset management businesses of the two institutions.

Japan's largest banks, which have emerged relatively unscathed from the US sub-prime mortgage collapse, told The Times earlier in the year that they stood ready to become the “silent investment partners” of any large Western banking names that came to them for help.

Sources at the top three Japanese banks said at the time that they had readied a $10 billion cash pile to effect precisely the sort of capital injection believed to be under negotiation between SMBC and Barclays. Mizuho Financial pumped about Y130 billion of capital into Merrill Lynch in January.

The Japanese deal follows confirmation from Barclays this week that it was considering a private placement and pre-emptive offer to restore its capital ratio in the aftermath of the American sub-prime mortgage debacle. Barclays has been hit by about £2.5billion in writedowns arising from the US mortgage crisis and resulting credit crunch.

At about 5 per cent, Barclays has one of the lowest core equity Tier 1 ratios - used as a measure of financial strength - in Europe. Rivals HBOS and Royal Bank of Scotland (RBS) want ratios of at least 6 per cent. HBOS is raising £4 billion through a rights issue to bolster its capital cushion, while RBS has raised £12billion from shareholders.

But Barclays is thought to want to avoid the drawn-out rights issue process, during which its rivals' share prices have fluctuated wildly.

Yesterday, Barclays' stock closed down 2.4 per cent at 308p.

Japan's capital markets are rapidly emerging as the favoured safe-haven for stricken US and European banking names. This year has already broken records for so-called Samurai Bond issuance - yen debt issuance by foreign firms - with names such as RBS, Citigroup and UBS all understood to be planning large capital raisings on the Japanese debt market.

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