The NHS is always an emotionally charged subject, and FactCheck’s inbox is full of warnings from campaigners about a supposed threat to Britain’s public health system from across the Atlantic.

Recent statements by US president Donald Trump and Woody Johnson, the US ambassador to the UK, have been seized on as evidence that the US is seeking to open up the NHS to US companies with a new bilateral free trade agreement.

Is there real cause for concern, or have the warnings been overcooked? Let’s take a look at what we know and what we don’t know.

How long will a trade deal take?

According to the Peterson Institute for International Economics, it has historically taken the US an average of three-and-a-half years to negotiate and implement bilateral free trade agreements with various countries, although there is huge variation.

Britain hasn’t negotiated a trade deal independently of the EU for decades and appears to be unprepared for talks with the US.

While the American government has already published its negotiating objectives, no equivalent document has emerged from the British side. This suggests Britain’s trade negotiators haven’t yet decided what they actually want to get from a deal.

With Donald Trump in the White House, there is an added element of uncertainty about a future trade deal.

The current president abruptly pulled out of negotiations over the proposed TTIP deal with the EU, has failed to get his new trade deal with Mexico and Canada through Congress, and is threatening the Mexicans with tariffs despite just agreeing a free trade deal with them.

Some expert commentators have suggested that a deal will never happen – or certainly not while President Trump is in the White House – but this is of course pure speculation.

What do the Americans want?

The US negotiating objectives document runs to 17 pages but only contains one explicit reference to medicine, saying America wants to “ensure that government regulatory reimbursement regimes are transparent, provide procedural fairness, are non-discriminatory, and provide full market access for US products.”

It’s not yet clear what this means in detail, but the focus on pharmaceuticals is potentially worrying, because President Trump and his Health and Human Services Secretary Alex Azar have threatened to use trade talks to try to push up drug prices outside America.

Their argument is that US pharmaceutical companies often charge American customers much more than clients in Europe and elsewhere.

It’s because large state-funded health services like the NHS have massive bargaining power. They buy drugs in enormous amounts and are good at negotiating discounts from the pharma companies.

In America, healthcare provision is much more fractured and there is no central agency bargaining with the drug companies, so they often get away with charging higher prices.

The Trump administration describes this as foreign countries “freeloading” at America’s expense. It reasons that US drug companies have to charge US patients more because their profits are being squeezed outside America, thanks to public health systems like the NHS.

So unusually, Mr Trump doesn’t just want US consumers to pay less for their drugs – he wants people in other countries to pay more, in the belief that this is the key to cutting prices in America.

All of this is a concern for some trade experts, because they think post-Brexit Britain will be the underdog in trade negotiations with the US, and might be compelled to take a hit on drug pricing to get an overall deal across the line.

The country with the bigger market usually has the upper hand and tries to put its own interests first, and there is little indication that President Trump will try to do Britain any favours on trade.

So the fear is that President Trump could use trade negotiations to try to force the NHS to pay higher prices for American drugs.

That’s not the same thing as individual British patients having to pay for drugs that they now get for free on the NHS, but it would obviously have serious implications for the NHS budget.

Could this lead to the privatisation of the NHS?

It’s hard to see how a future trade deal could fundamentally undermine the principles of the NHS.

Britain’s health service is (mostly) free for patients at the time of use and funded from general taxation, and it’s not clear why a trade agreement would change this, or lead to the adoption of an American-style system, based on private insurance.

US politicians or companies are not in a position to dictate how the NHS is run, and other recent trade deals have often included clauses specifically stating that the agreement will not affect the ability of domestic governments to administer public services.

When the TTIP negotiations were underway, the EU said it was working on the principle that no agreement could force a member state to open public health services to competition from the private sector, or prevent them from bringing outsourced services back into the public sector.

It’s not clear that the US government would object to similar carve-outs for the NHS, if the British demanded them. Statements made by President Trump and Ambassador Johnson about the NHS have been ambiguous.

In a press conference on Tuesday, the president said: “When you’re dealing with trade everything is on the table, so the NHS or anything else, or a lot more than that.”

But after an angry reaction from British politicians he rowed back on this a day later, saying: “I don’t see it being on the table” and adding that the NHS was something he would “not consider part of trade”.

Ambassador Johnson also sparked outrage when he appeared on the BBC’s Andrew Marr show and was asked whether “healthcare has to be part of the deal”.

The ambassador replied: “I think probably the entire economy, in a trade deal all things that are traded will be on the table.”

It’s worth noting that Marr used the word “healthcare” in this exchange, not “the NHS”, which leaves open the possibility that Ambassador Johnson might have taken the question to refer to the private healthcare sector.

In an earlier exchange he was asked about “American pharmaceutical and medical corporations getting a slice of the NHS” and replied: “Oh, I don’t think they’d be put in any particular position regarding that. Your national healthcare service is the pride of the country.”

Is the NHS already being privatised?

The NHS has always involved private companies, and there’s a bit of confusion about what people mean when they talk about businesses taking “a slice of the NHS”.

The health service spends enormous amounts of money buying goods and services from private suppliers, from hand sanitisers on wards to complicated medical instruments and whole areas of service delivery.

American companies already have the right to bid for contracts and some are heavily embedded in the NHS supply chain.

It’s hard to say whether a free trade agreement would lead to more contracts for US private healthcare firms. Even if it did, this would not represent a change in the way the health service is run: care would still be free for NHS patients.

The NHS has been outsourcing services to the private sector since Tony Blair welcomed bids from independent providers in the 2000s.

Since then, the share of contracts given to private companies has risen, but the extent of private provision in the NHS is often overstated.

The NHS now pays around £9bn a year to private companies to deliver services on behalf of the taxpayer.

In 2017/18 that represented 7.3 per cent of day-to-day health spending, down slightly from 7.7 per cent in the previous financial year.