The promotion marks a swift return for the two companies as they were only demoted at the time of the last FTSE reshuffle in March.

Logistics group Hays and outsourcing specialist Capita have been relegated from the index.

The composition of the FTSE index is reshuffled every three months to make sure it represents the UK's largest publicly listed companies.

A company's stock is promoted into the FTSE 100 if the firm's market capitalisation ranks in the top 90 at the close of business on the last day of trading before the review - in this case Tuesday.

If a stock is ranked 111th or lower, the company is ejected from the index and it drops into the FTSE 250.

The changes are based on Tuesday's closing prices, but were approved at a meeting on Wednesday.

Narrow escape

The pub group Mitchells & Butlers - which owns the All Bar One and Harvester chains - had been tipped as another candidate for relegation.

But the firm was saved after positive broker comment led to its shares rising 3% on Tuesday.

The move was enough for the firm to avoid relegation from an index it only entered in April following its demerger from hotels group Six Continents.

Despite the much-publicised problems at steel maker Corus the firm has won promotion to the second-tier FTSE 250 index.

Corus' shares have risen from 6p in March to 19p as talks over its finances continue.

Other companies to win promotion to the FTSE 250 include Iceland owner Big Food Group, online travel agency Ebookers, engineering and property services group WS Atkins and telecoms group Thus and IT firm Spirent.

They replace Law Debenture Corp, Euromoney Institutional Investors, Beazley Group, Aquarius Platinum, Shanks Group and Nestor Healthcare Group.