Embattled security firm G4S saw shares come under further pressure today after lower-than-expected interim results and confirmation of an expected £50 million hit from the Olympics contract fiasco.

The stock fell 2%, down 4.7p to 261.6p, after reporting a sharp fall in half-year pre-tax profits to £61 million from £151 million a year earlier in interim results that were dominated by its London 2012 security staffing debacle.

The wider FTSE 100 Index was also in the red - down 20.8 points to 5755.8 - as investors returned after the bank holiday weekend amid uncertainty over whether the European Central Bank will act swiftly to tackle the eurozone debt crisis.

Marks & Spencer joined G4S on the fallers board as it slipped back as last week’s excitement surrounding rumours of an £8 billion bid from CVC Capital Partners began to fade.

Shares in the retail giant were down 8.2p to 363.5p today.

B&Q parent Kingfisher was another retailer suffering falls, down 3% or 9.3p to 276.4p, following a broker downgrade.

Elsewhere, defence contractor Chemring plunged 12% or 45.2p to 325.3p in the FTSE 250 after the company warned over full-year profits after production delays and errors in a new planning system.