Telecoms firm Cable & Wireless Worldwide and engineer Invensys are among those likely to be relegated from the FTSE 100 in the latest quarterly reshuffle.
Both groups have recently received a boost from takeover speculation, although this is unlikely to save them from slipping into the second tier based on their closing prices this evening.

Argos owner Home Retail Group is also set to find itself stripped of its premier league status after 21 months in the top index.

Its shares have lost more than 25 per cent in value after its latest trading update disappointed investors as consumers continue to rein in their spending.

In contrast, engineer Weir Group and Clive Cowdery's Resolution are likely to be promoted along with engineering group Tomkins.

However, Tomkins will only stay in the index for a short time having agreed a £2.9 billion takeover earlier this week.

Weir recently said it expects profits to be 'significantly ahead' in the second half following a 24 per cent rise in order input in first-half trading. It has also been expanding its global reach of late with the acquisitions of Malaysian group Linatex and Indian firm BDK Engineering Industries.

Companies can join the FTSE 100 if they are among the top 90 UK-listed companies based on market capitalisation. However, they can be pushed down to the FTSE 250 if they fall to position 111 or below.

Meanwhile, Resolution, the insurance vehicle behind Friends Provident, should re-enter the top flight index after slipping from grace back in March.

Among the smaller players, online grocer Ocado is set to make its debut in the FTSE 250 after floating last month at 180p a share. Although its flotation proved less successful than initially anticipated, its shares are changing hands at around 149p, giving it a market cap of around £800 million.

Troubled social housing group Connaught will tumble off the second tier index after its shares shed more than 90 per cent of their value since it issued a profit warning over the impact of spending cuts at the end of June. It had a market value of around £24 million before its shares were suspended today.

Other companies likely to be feeling the pain of relegation later on today include renewable energy firm Eaga, educational whiteboards maker Promethean World, diamond producer Gem Diamonds and video games and consoles retailer Game Group.

These will all drop into the FTSE All-Share and are likely to be replaced by chemicals firm Elementis and car hire business Avis Europe.