The £400m sale is expected to take place shortly after 3i reveals its 1994 results on 8 June, and will go mainly to institutional investors in one of the biggest ever secondary issues on the London market.
Originally known as Investors in Industry, 3i has for some 20 years been a leading provider of long-term finance to companies throughout the UK - particularly medium-sized manufacturing businesses.
The cashing-in on 3i's strong share performance, which had been expected for some time, will reduce the stake held by the big banks to about 32 per cent. Exact details of the sell-off have not yet been finalised, nor whether some of the banks will sell out completely or retain a portion.
The Bank of England is widely expected to part with all its 6.6 per cent. The biggest single investor, NatWest Group, yesterday said it would retain its 18 per cent stake, as will the Bank of Scotland with its 2.5 per cent.A NatWest spokesman said the "bank is very comfortable with its investment and the sector. We see no reason to change the nature of our shareholding." Both NatWest and Bank of Scotland said they do not intend to sell other 3i shares during 1995.
Since the flotation last July, when the banks first reduced their holding from 97 per cent to 52 per cent, 3i shares have outperformed the all-share index by 25 per cent, making them one of the FT-SE 100's best performers. The shares, floated at 272p, closed yesterday down 9p at 346p.