Index provider FTSE confirmed on Wednesday that Yell would replace water firm Kelda in the UK's premier share index on 22 September.

But British Airways has missed out on promotion after seeing its share price fall back in recent days.

Meanwhile, Marconi Corporation has gained promotion to the FTSE 250 index.

Maiden entry

In August, Yell announced that its losses had more than doubled during the second quarter of the year.

But it blamed its losses on the cost of its July public float, and has indicated to shareholders that it is on track to meet full-year targets.

Its promotion to the FTSE 100 means it is now the second former BT firm, after MMO2, to enter the top flight of UK companies.

Missing out

Hopes had risen that the reshuffle would also see British Airways re-entering the FTSE 100.

Its share price had risen in recent months as air passenger figures began to recover from the lows of the war in Iraq and the Sars virus outbreak.

But its shares fell back this week following comments from chief executive Rod Eddington that recovery was still some way off.

News of a new case of Sars in Singapore also rattled investor confidence in the air sector.

FTSE 250 changes

While the FTSE 100 only saw one change, there was far more activity in the second tier FTSE 250 index.

The most notable promotion to the index was that of Marconi Corporation, the new guise of the telecoms equipment firm that underwent a major financial restructuring earlier this year.

In addition to Marconi, other firms to be promoted included reinsurance broker Benfield, mining firm Randgold, technology firm BTG, photobooth operator Photo-Me International and publisher Euromoney Institutional Investor.

Heading in the other direction were logistics group Tibbett Britten, telecoms firm Kingston Communications, Cox Insurance, oil and gas firm Soco International and two investment trusts - Candover Investments and Perpetual Income & Growth Trust.

Key index

FTSE reshuffles its benchmark index each quarter to ensure that the UK's largest firms are represented.

Membership of the FTSE 100 is important, as some fund managers and pension funds will only invest in the shares of firms in the flagship index.