LONDON (BLOOMBERG) - A few hours in the genteel English countryside next week could be the biggest risk to Boris Johnson in the final stretch of his election bid.

The British prime minister will interrupt his campaign to briefly host Nato leaders at a luxury resort with a golf course and spa. Despite the tranquil surroundings, those close to Johnson fear the gathering at the Grove Hotel in Hertfordshire could turn into a train wreck that hurts his bid for power.

The potential catalyst for their nightmare scenario? Johnson's close ally, Donald Trump.

Ahead of the Dec 12 election, the US president may yet become the most dangerous weapon in the armoury of British opposition leader Jeremy Corbyn, whose Labour Party aims to oust Johnson's Conservatives.

Corbyn's argument is simple: Johnson and Trump are right-wing soul mates who collectively pose an unprecedented threat to the UK's cherished National Health Service.

If Johnson wins a majority in Parliament and completes the UK's exit from the European Union, he will rush towards a free trade deal with the US in which the NHS will be up for sale, Corbyn says.

That, he claims, will lead to higher prices for drugs and ruin forever the free-to-use healthcare that Britons have come to expect.

Despite denials from the Conservatives that the NHS would be on the chopping block, it's a potent narrative with voters - one that has troubled Johnson's senior team since long before the election campaign started. And now Trump will materialise on British soil just 10 days before the election.

Trump lands in London on Monday. He'll be greeted by the usual anti-Trump protesters, and Corbyn is certain to make the most of any pictures of Trump and Johnson standing shoulder to shoulder.

Privately, though, what senior Tories worry about the most is that Trump will go off script. One senior official confided that this was the biggest risk facing the election operation. The president can't be trusted to stop himself weighing in with ill-judged comments, the official said.

A Cabinet minister endorsed those concerns. Another senior official said measures were being taken to minimise the risk, though they didn't say how this would be possible given Trump's penchant for speaking his mind at all hours on Twitter, or for going off topic when he holds media briefings.

And there are good reasons for the Conservatives to be anxious.

On his previous visit to London in June, Trump gifted Corbyn an attack line by suggesting at a briefing with Johnson's predecessor Theresa May that access to the NHS for US companies would be one of the things "on the table" in trade talks after Brexit.

Following the inevitable outcry, Trump issued a clarification, saying the NHS was not something he'd be seeking to exploit. But the damage had been done.

Labour received another boost on Oct 31 when Trump gave a surprise interview to his friend Nigel Farage, the veteran far-right politician who leads the Brexit Party. He ditched the convention of political neutrality in a foreign election to warmly endorse the "fantastic" Johnson, while warning Corbyn would be "so bad" for the UK.

During the interview, Trump urged Johnson and Farage to join forces to defeat Corbyn and complete Brexit. He also tried to make amends for his earlier comments by saying he did not want the NHS to be on the table for a future trade deal.

"I think Trump actually pole-axed Corbyn," Farage said in an interview this month. "Corbyn says the NHS will be sold to the Americans, Trump says it's not even on the table."

Yet Tory advisers were aghast. By mentioning the NHS again, even in a denial, the president ensured it stayed prominent in the public consciousness.

And while Farage is a fan, there is some evidence that Britons increasingly are not. Most people questioned by YouGov earlier this year saw a trade deal with the US as worthwhile, but a separate survey found that fewer thought the government should try to work with Trump than did so when he first became president.

According to YouGov polling conducted since the start of the election campaign, the NHS is the single most important domestic policy priority for voters.

While Brexit is the top issue for the country, healthcare is second. It is a measure of the sanctity of the NHS that more than half of ordinary taxpayers (53 per cent) would be willing to pay more to fund investment in it. Fewer than a third (31 per cent) of people opposed higher taxes to do so, the research showed.

Founded 71 years ago, the US$200 billion (S$273 billion) a year NHS has become a rallying point among Labour supporters and left-wing politicians.

Corbyn regularly addresses rallies of healthcare workers. Doctors and nurses have urged people to vote against the Conservatives to protect the NHS from privatisation or funding cuts.

Since Johnson became prime minister in July, he has been on a mission to recover lost ground on the NHS. Barely a week has passed without a visit to a hospital, sleeves rolled up and tie tucked for hygiene purposes into his shirt. But senior Conservatives are concerned he remains vulnerable to Labour's attacks.

One Cabinet minister said that even in a safe Tory seat, Johnson is not wholly trusted, with voters likely to question his claims to be spending billions on building new hospitals. Another warned that whatever the truth of Labour's arguments, voters are listening to what Corbyn is saying.

The minister was furious at publicity given to Corbyn's claim this week to have unearthed a document purportedly showing a secret government plan to sell the NHS, when Tories say the paper proved nothing.

According to the minister, the Conservatives' internal polling shows voters don't buy Corbyn's argument the NHS would be for sale if Johnson struck a trade deal with Trump. But the Labour leader's point has still cut through. Voters know the NHS debate is not finished.

If, as expected, Corbyn tries to use the Nato summit to attack Johnson for his links to Trump, Tories plan to retaliate: By stressing the Labour leader's weakness on national security (he's a lifelong anti-war campaigner) and his reluctance to support Nato.

The Tories are taking other measures, too. When Trump arrives at the country estate, don't expect him to be welcomed with fanfare or a public hug from Johnson. A joint press conference is not on the agenda for now. This is one presidential visit that will be kept as low key as Johnson's nervous aides can manage.