As we all know, David Cameron is prone to hyperbole and gross exaggeration.

After the G8 summit in Northern Ireland our PM enthusiastically told the world that the EU-US free trade deal, the "Transatlantic Trade and Investment Partnership" (TTIP), would be worth trillions of dollars to both European economies and that of the United States.

We were told that 50 negotiators would meet in offices near the White House to put together an agreement that would create a million jobs in the US and 400,000 in Europe.

The European Union welcomed the whole process and set about copying many of the other trade deals that have been done with different countries around the world.

I think this is a time to be extremely wary of such agreements, however. Detailed negotiations are being carried on behind the scenes, while big business lobbying - also hidden from public view - is putting pressure on the negotiators.

In trade disputes national governments are often the last to influence any decision, as secret panels of arbitrators decide whether or not legislation passed to protect the social good oversteps the mark by interfering with the interests of business.

Trade agreements don't have to be bad things. But we must pay careful attention to the effect they have on social and employment legislation.

The European Commission says TTIP is to be the biggest trade deal in the world. Its purpose is supposedly to drive growth and create jobs.

It will, apparently, expand the EU economy by €120 billion (£100bn), the US by €95bn (£80bn) and so on and so forth. At no stage has anyone been specific on how this is to happen.

Shortly after the process was announced the Centre for Economic and Policy Research's Dean Baker wrote: "As growth policy, this trade deal doesn't pass the laugh test, but that doesn't mean that it may not be very important to a number of special interests and for this reason bad news for most of the public.

"Since conventional barriers to trade between the US and EU are already very low, the focus will be on non-conventional barriers, meaning various regulatory practices."

This week the negotiators are holding sessions dealing with services, investment, energy and raw materials.

Tariffs on goods being moved between the EU and US are generally around 3 per cent at the moment. The removal of all tariffs would not have an enormous effect on trade.

What might have that effect is changing the regulations on exporting and importing goods, the "rights" of companies such as airlines to access each others' markets and of course the key issue of social expenditure levels and workplace rights.

Generally speaking Europe has much better environmental and recycling standards than the United States, which relies on market mechanisms to "regulate" these areas with woeful results. This could become important if regulations about acceptable emissions levels in cars, say, were deemed to interfere with the right of US car companies to trade freely in Europe.

A crucial area of negotiation is over agriculture and food standards. There is wide use of genetically modified crops in the US. These can have very high yields but leave a wake of damaged biodiversity, impoverished soils and poorer ecosystems.

We should be very suspicious of the power of the agribusiness lobby to develop a cosy agreement between the EU and US which is then presented as a fait accompli to legislators on both sides of the Atlantic.

The negotiations will take place over two years, and since they will be conducted in secret we should be asking precisely who is sitting round the table.

In July the European TUC called for the negotiations to enshrine core labour standards, ensure a trade union role in dispute resolution and integrate social partners in the actual talks and in monitoring the results - that is, give labour representatives a voice.

The ETUC also called for public goods to be excluded from intellectual property measures and for government's right to regulate in the public interest and favour the public delivery of services to be protected. Indeed, it asked for public services to be kept out of the talks entirely, as well as agriculture.

The talks should promote environmental protection, maintain EU standards on the environment, co-ordinate action against tax avoidance and tax havens via a transatlantic financial transactions tax, ensure external investors have no right to bypass European courts and restrict themselves to specified areas, with some topics - further liberalisation of the financial sector, for example - out of bounds, the unions said.

But the labour movement is unlikely to get a sympathetic hearing from the negotiators. Britain's TUC general secretary Frances O'Grady warns that the deal could "open up public services to privatisation on an unprecedented scale."

The TUC continues to demand core labour standards and trade union involvement in the negotiations.

The American Federation of Labour and Congress of Industrial Organisations (AFL-CIO) has noted that increasing trade ties with the EU could be beneficial for US and European workers - "but as with all trade agreements, the rules matter."

US unions are right to be concerned. Workers' rights are not even uniform across the 50 states of the US, while under the North American Free Trade Agreement (Nafta) US companies are easily able to exploit workers in Mexico, creating a desperate race to the bottom which damages organised US workers as well as poor migrant workers on either side of the border. In its day Nafta too was flagged up as a job-creating deal, but the predictions made for it in this regard proved wildly inaccurate.

In a period of austerity the agenda of the European Central Bank (ECB) and of banking interests in the US is to claim that good wages and working conditions, and the trade union rights which maintain them, are damaging to economic growth.

Actually regulated and organised workforces tend to be more productive than those being forced to sacrifice their rights in the way the ECB has done to those of Spain and Greece.

Workers' rights across Europe have been won by trade union action influencing the political process. Health and safety legislation, for example, was a victory for the trade union movement after many years of campaigning.

We must be extremely wary of the planned trade deal. Many of those negotiating it are committed to destroying agreed work practices, reducing or removing the influence of trade unions and the social protection they provide.

They are also devoted to mass privatisation, something the EU is already trying to force through with its railways directive.

And private health companies are lining up to destroy state-run health services to ensure that the private health sector takes over.

Only by extreme vigilance can the labour movement ensure that the elites of two continents don't combine to undermine everything workers on both sides of the Atlantic have fought for over the decades.

Jeremy Corbyn is Labour MP for Islington North