Marconi has sold its North American Access (NAA) unit to US-based Advanced Fibre Communications for around $240m (£133m) in cash.
The move looks set to lead to a reduction in the UK equipment maker's debt mountain and has led to a more positive statement about the company's upcoming third quarter figures.
Marconi's NAA business is Ebitda positive, recording a profit of about £3m on sales of £55m in the six months ended 30 September 2003.
The deal is expected to close before the end of this calendar quarter and is thought to have been finalised quicker than expected.
Marconi was advised on the transaction by JP Morgan.
It is left with around £377m in debt and a market cap of around £1.26bn after a slight rise in its shares this morning -- down from a high during the Internet bubble of £35bn.