Branding can be horribly misleading. The official logo of the Salvation Army, a charitable organisation that these days deals mainly in jumble sales, involves crossed swords and the slogan "Blood and Fire". Meanwhile, G4S, the world's largest private security company, whose operatives provide hired muscle to asylum detention centres in the UK, private prisons in America and government facilities in West Bank settlements, has just a neat black-and-red slash and the words "Securing your world". It even has a jolly theme tune, an apparently unironic track called "G4S: securing your world", which involves pounding synths and teeth-clenching rhymes like "let your dreams unfurl". It's hard to say whether this has done more damage to the company's reputation than the case of Jimmy Mubenga, an Angolan asylum-seeker who died while being deported by G4S employees in 2010.

I mention all this because G4S will shortly be patrolling the London Olympics with more than 10,000 private security agents. The British-based company, billing itself as the "world's leading international security solutions group", will be the main provider of all manner of surveillance services to the Games, which will all cost hundreds of millions to the British taxpayer – a bill which has tripled from original estimations. Questions are being asked in Parliament about G4S's human rights record, but the biggest question has yet to be raised: are we really happy for global security, from prisons to police, to be in the hands of private firms that turn immense profits from the business of physical enforcement and are accountable almost exclusively to their shareholders?

The first thing you need to know about G4S is that it's enormous. It has 657,000 employees – more than the population of Glasgow – and is the world's second-largest private employer, after the American retail giant WalMart. It's also booming, with profits up 39 per cent in 2011. In Britain, G4S is the recipient of hundreds of millions of pounds' worth of government contracts, which go way beyond the Olympics. G4S operates prisons and asylum centres across the UK, and will be moving into policing as more and more public services are cut. The company is, in fact, one of the main financial beneficiaries of the Coalition Government's privatisation drive as the state seeks to divest itself of various expensive responsibilities.

The Government has continued to hand out lucrative contracts to G4S, despite the fact that it lost one contract following complaints, though G4S said the reason was cost. Complaints about G4S's deportation service culminated in the arrest of three employees over the death of Jimmy Mubenga. G4S whistleblowers had already given secret evidence to a parliamentary select committee about potentially lethal techniques they said were being used to restrain asylum-seekers, including so-called "carpet karaoke" – stuffing a deportee's face towards the floor to contain them.

This is the new face of the global for-profit security business. Outside the UK, G4S operates in 125 different countries, including Iraq, Afghanistan and Israel, where its paid agents operate checkpoints and provide security at jails for Palestinian prisoners, including child detainees. Israel, where the company's turnover is £120m, will be the subject of the question to be raised in Parliament on Monday regarding whether the presence of operatives in occupied Palestinian territories, including in prisons that hold children, violates the terms of the Olympic charter. Palestinian prisoners and terrified asylum-seekers do not appear on any of G4S's promotional material, but there is lots of footage of smiling people in uniform standing near sporting events.

Technically, we are not allowed to call these people mercenaries. "Mercenary" has a specific definition under the terms of the Geneva Convention, including technical conditions like being born outside the country of operation, which happens to exclude nearly everyone working for a for-profit security firm. Instead of the more loaded term, we must call them private security employees, but we could call them the Happy Fun Henchman Club and there still wouldn't be enough national or international law holding them to account.

Nor does there seem to be any great anxiety to put those laws in place. In Britain, private security agents might be hired to do the same jobs as police officers and prison guards, but they're not accountable to the public in the same way – at least, not yet. The Independent Police Complaints Commission still has no power to investigate private security staff, and the Government is prevaricating over the watchdog's request to extend its remit – which was supported by G4S – while extending the outsourcing of policing to for-profitcompanies. G4S was recently awarded a £200m contract to take over half of the civilian duties of Lincolnshire police force. Policing employees helping protect the public in Grimsby and Scunthorpe will now wear G4S's company logo – that discreet sharp slash of red and black.

What difference does it make if the men and women in uniform patrolling the world's streets and prison corridors are employed by nation states or private firms? It makes every difference. A for-profit company is not subject to the same processes of accountability and investigation as an army or police force which is meant, at least in theory, to serve the public. Impartial legality is still worth something as an assumed role of the state – and the notion of a private, for-profit police and security force poisons the very idea.

The state still has a legal monopoly on violence, but it is now prepared to auction that monopoly to anyone with a turnover of billions and a jolly branding strategy. The colossal surveillance and security operation turning London into a temporary fortress this summer is chilling enough without the knowledge that state powers are being outsourced to a company whose theme tune features the line: "The enemy prowls, wanting to attack, but we're on to the wall, we've got your back." If that made any sense at all, I doubt it would be more reassuring.