Donald Trump will put the interests of corporate America first and demand that the NHS pays higher prices for US drugs in a free-trade deal with the UK, the outgoing British ambassador to Washington has told the Guardian.

Kim Darroch, in his first interview since his resignation from his post in July, from where he spearheaded attempts to grow trade with the US, insisted that Trump would reward his backers in drug firms and farming communities by opening up British markets, while questioning where the UK’s gains would be found.

The former British envoy, who left Washington after a leak of his confidential cables to London, said it was doubtful whether the UK had the resources for parallel negotiations with the US and the EU, a strategy championed by Downing Street as a way to give British negotiators leverage in Brussels.

Darroch, who said that warnings on the US’s trade demands had been made to No 10 during his tenure in Washington DC, also said it was “impossible” for a deal to get through Congress by the end of 2020 and that it appeared to be “a narrow and rocky path to get to where they [the UK government] want to be”.

He said: “I know what the US will be pitching for when they negotiate a free-trade deal with us. They will pitch for massively greater access for agricultural products. People talk about chlorinated chicken – it is a lot more than that. Farmers in America vote for Trump, pretty much all of them vote for Trump …

“They also want us to pay the same for American pharmaceuticals as they pay in their own market. Do they want us to pay more for their pharmaceuticals? Do the pharmaceutical companies want to use this leverage? Of course they do.”

Play Video 1:09 UK 'at front of line' for US trade deal, says Pompeo – video

The independent National Institute of Health and Care Excellence sets a price cap on drugs used in the NHS which forces US drug firms to sell their products more cheaply in the UK. During the general election campaign, Boris Johnson was forced to deny claims from Jeremy Corbyn that sales to the NHS would be part of any trade negotiations.

Darroch dismissed Labour’s soundbite that the NHS would be “up for sale” but he stressed that the president would be focused on pleasing his voters. “He believes in America first,” Darroch said. “And he believes, particularly, in rewarding people who vote for him and that is American farmers and big American corporations.”

Darroch’s intervention came as Mike Pompeo, the US secretary of state, conceded that the talks with the American negotiators would involve “hard conversations” and warned the British not to use “food safety as a ruse to try and protect a particular industry”.

On the prospect of parallel negotiations as the UK leaves the EU, Darroch further questioned whether Trump would want to focus on the talks in an electoral year but he confirmed the strategy had been discussed at the highest levels of government.

Darroch, who was made a cross-bench peer in Theresa May’s resignation honours list, said: “They have talked about it a long time, even during Theresa May’s time. It is one of those things that sounds logical in theory: that you balance one against the other and there is a logic to outside leverage.

“But you are going to have to make a choice between whether you go for European standards and rules and all the rest of it, or whether you go in some areas with American ones … I can see the logic. But it is going to be a hell of job just in terms of sucking up resources. We have not done trade negotiations for 40 years. It is also going to be an electoral year in America and I don’t quite see how that is going to work.”

Darroch, a former national security adviser and permanent representative to the EU, said: “I am not sure president Trump is going to be massively focused on this trade negotiation while he is fighting for a second term and therefore whether the US trade representative is going to get quite the direction they need from the White House. So I am sceptical about the logistics and resources required.”

The UK is yet to publish its negotiating objectives in trade talks with the US. “Frankly because we haven’t had the bandwidth to work it out,” said Darroch.

The biggest success story for British exports to the US has been in the sale of luxury cars, said Darroch, but the tariffs on those were already low, making the potential gains relatively minimal. He said: “What we would love is much better access on financial services, maybe we will get that but I don’t know what that means.

“British Airways would love to be able to pick up passengers in London, fly them to, say, New York or Boston, and fly them on to St Louis and finish up with them in San Francisco. Are we going to get that? There is no chance the Americans are going to give us those rights to allow British carriers to break into the US domestic market, which is fantastically lucrative.

“We would love to be able to get into state and federal procurement markets. Are they going to let us into that? I don’t think so. We will no doubt ask for it.

“We still can’t sell really serious military kit to the Americans. There are missiles that we make that are better than anything they have in terms of precision. But we can’t sell them because the US industry has it stitched up. Are we going to get access on that? We will see. I just wonder where these huge advances are going to be on a US trade deal.

“Maybe Boris’s relationship with Donald Trump is so fantastic that Trump will give him all this as a gift,” Darroch said. “But are they going to while we’re saying we are not going to have chlorinated chicken, not going to have your hormone-treated beef, not going to have all your genetically modified crops and we are not going to pay twice as much as we do now for American pharmaceuticals.”

Darroch resigned in the summer after leaked memos revealed that he had described Trump’s administration as “clumsy and inept”. Trump tweeted in response that he would not work with the British ambassador. A police investigation into the leak is ongoing. Officers currently believe it was not the result of a hack into government systems.

A Department of Health and Social Care spokesperson said: “The Government has been clear that when we are negotiating trade deals, the NHS will not be on the table. The price the NHS pays for drugs will not be on the table. The services the NHS provides will not be on the table. In any negotiations on future trade agreements we will never include any proposals that would put NHS finances at risk or reduce clinician and patient choice.”