National Grid will offer between 71 million and 75 million Energis shares in the form of shares and convertible bonds to institutional investors in Europe and the US. Dealings are expected to start once the offering closes next Tuesday.
The move will raise approximately pounds 1.1bn in cash, allowing National Grid to reduce its debt levels in the wake of last month's $3.2bn acquisition of New England Electric System, the US electricity distributor.
David Jones, chief executive of National Grid, said the sale would also increase the free float of Energis shares, satisfying demand from investors keen to buy into the telecom sector and allowing the company to be included in the FT All-Share index.
Energis shares duly soared 9 per cent, closing up 132.5p at 1655p in a falling market. Energis has watched its shares rise fivefold in the past 12 months on the back of booming data communications and Internet traffic, including its deal to provide the infrastructure for Dixons' innovative Freeserve Internet service provider.
The sale follows National Grid's announcement last year that it planned to sell down its shareholding in Energis over a three to five-year period. Under the current deal, it is not allowed to sell any more shares in the market for six months.
In a complex offering designed by HSBC, the investment bank, National Grid plans to raise between pounds 600m and pounds 800m by placing Energis shares directly with institutions. The remainder of the proceeds will come from the sale of convertible bonds based on Energis shares.
Each bond, which will mature after four years, will convert into a proportion of an Energis share dependent on how much the shares have risen since the offering. However, they will also carry a coupon of between 5 and 6 per cent, which will be paid by National Grid.
Mr Jones said the bonds were designed to broaden the appeal of Energis to other investors, many of whom do not invest in the company because it does not pay a dividend.
The entire exercise is expected to trigger a capital gains tax bill of about pounds 200m for National Grid, while advisers' fees will cost a further pounds 30m.
Outlook, this page