Embattled private security giant G4S is scrambling to raise up to £600 million by selling shares and businesses in a bid to slash debt and stave off a ratings downgrade.

The company, which is still reeling from its Olympics security failure and an overcharging scandal for tagging offenders, is selling new shares worth around £350 million at last night's prices, plus offloading businesses for a total £250 million.

G4S, which has an army of 620,000 staff across the globe, said it will also restructure under plans to reduce a debt mountain worth almost £2 billion at the end of June.

The biggest employer on the FTSE 100 Index slumped to half-year pre-tax losses of £87 million for the first six months, after profits of £48 million a year earlier were wiped out by a £180 million writedown of assets and liabilities.

New boss Ashley Almanza said G4S must strengthen its balance sheet as he revealed the sale of about 141 million new shares, equivalent to 10% of its existing equity base.

Mr Almanza replaced Nick Buckles in June, after the former chief executive came under fire for its botched handling of its Olympic Games contract.

Its Olympics failure saw Mr Buckles hauled before MPs, during which he admitted it was a "humiliating shambles for the company". Extra military personnel had to be called in to fill the gap left by G4S's failure to supply enough staff for the £284 million contract.

G4S, together with rival Serco, was also mired in further controversy around an overcharging scandal which among other errors saw the Government billed for tagging offenders who were back in prison, had had their tags removed, or in some cases, were dead.

Today's cash call marks a steep fall from almost two years ago, when G4S announced a £5.2 billion takeover of Danish facilities management firm ISS.

The deal, which was later scuppered by shareholders, had aimed to create a company with around 1.2 million staff and revenues of £16 billion.

G4S today warned a ratings downgrade could add up to £30 million a year to its debt costs.

It is selling a Canadian cash solutions business and a US data storage company for a total £100 million, which combined with other announced US sales and imminent deals will recoup a total £250 million.

Mr Almanza said: "We need to strengthen our balance sheet to be able to realise the group's opportunity for substantial value creation.

"2013 will be a year of consolidation for the group with the actions we are now taking starting to deliver tangible benefits during 2014."

The company also announced the appointment of Himanshu Raja, current chief financial officer at software group Misys, as its new finance chief. He will start in October.

Half-year revenues grew 7% to £3.65 billion year-on-year, including 5.4% organic growth. The company said it has a "strong and growing" pipeline of contracts worth £4 billion a year.