The UK PM has suggested that British intelligence sharing could be withdrawn from some operations with the US, if torture is reintroduced.
US President Donald Trump has said he believes waterboarding works, stating “we have to fight fire with fire”.
The UK military cannot use information obtained by torture or join operations where it might take place.
Theresa May said: “Our guidance is very clear about the position… and our position has not changed.”
BBC political editor Laura Kuenssberg said it implied that, were the US president to allow torture again, huge amounts of joint working would have to end.
Mrs May has been urged to reject President Trump’s comments about torture when she becomes the first world leader to meet him.
Speaking to journalists on a plane on her way to the US, Mrs May was asked about the new president’s comments about torture.
She replied: “We condemn torture and my view on that won’t change – whether I am talking to you or talking to the president.”
But she defended her eagerness to visit the White House early and meet Mr Trump: “Donald Trump was elected president of the USA.
“The US and the UK have shared challenges, shared interests that we can work together to deal with.
“We have a special relationship, it’s longstanding, it’s existed through many different prime ministers and presidents.”
Before her talks with the president Mrs May travels to Philadelphia to address Republican leaders at an event attended by the US president.
The issue of torture rose up the agenda after the president’s comments to ABC News on Wednesday.
Do opposites attract?
By Laura Kuenssberg, BBC political editor
As she made her way across the Atlantic, Theresa May joked with the press pack on her flight that “sometimes opposites attract”.
A wisecracking way of trying to cover the question about how she and Donald Trump can work together – the reality TV star billionaire and the self-described hard working vicar’s daughter.
Voters will decide for themselves how funny they find it.
But Number 10 has already invested a lot in the early days of this relationship.
He said: “When they’re shooting, when they’re chopping off the heads of our people and other people, when they’re chopping off the heads of people because they happen to be a Christian in the Middle East, when Isis (IS) is doing things that nobody has ever heard of since Medieval times, would I feel strongly about waterboarding?
“I have spoken with people at the highest level of intelligence and I asked them the question ‘Does it work? Does torture work?’ and the answer was ‘Yes, absolutely’.”
Mrs May said at Prime Minister’s Questions this week that the government’s opposition to torture remained unchanged, a message repeated by Foreign Secretary Boris Johnson and Brexit Secretary David Davis on Thursday when MPs asked about Mr Trump’s comments.
Opposition leader, Labour’s Jeremy Corbyn said: “Theresa May must stand up for our country’s values when she meets Donald Trump and oppose his support for torture, which is inhumane, illegal and delivers false intelligence.”
During her visit Mrs May will vow to renew the UK’s special relationship with the US “for this new age”, telling Republicans that after Brexit, a “sovereign, global” Britain wants to enhance ties with its “old friends”.
Post-Brexit trade opportunities, security and intelligence co-operation and the future of Nato are likely to feature significantly in her talks with Mr Trump.
Present for the president
Theresa May will present Donald Trump, whose mother was born in the Outer Hebrides, with an engraved quaich, a Scottish artefact symbolising friendship.
The shallow bowls were traditionally used in Scotland from the 17th Century as drinking cups.
First lady Melania gets a hamper of produce from Chequers – containing apple juice, damson jam, marmalade, Bakewell tarts and “cranberry and white chocolate shorties”.
Mrs May has said her goal is to build on the historic relationship between the two nations, underpinned by their shared values and common interests.
But she has said she “won’t be afraid” to speak candidly to Mr Trump on matters where they disagree, having criticised remarks the businessman has made in the past about women and Muslims.
The government is keen to capitalise on the US president’s strong personal and business links to the UK and his support for Brexit – which he has described as a “smart move”.
In her speech to the annual Republican retreat on Thursday, Mrs May will stress the unique contribution that the US and UK have made to the modern world and the institutions that underpin it, such as Nato and the United Nations.
She will say that just as the two nations have worked hand-in-hand to “defeat evil” and to “open up the world” in the past, they now have an opportunity to “lead, together, again”.
“The UK is by instinct and history a great, global nation that recognises its responsibilities to the world,” she will say.
“And, as we end our membership of the European Union, we have the opportunity to reassert our belief in a confident, sovereign and global Britain, ready to build relationships with old friends and new allies alike.
“So as we rediscover our confidence together – as you renew your nation just as we renew ours – we have the opportunity, indeed the responsibility, to renew the special relationship for this new age.”
Although the UK cannot begin to negotiate trade deals with the US or other countries until it leaves the EU, Mr Trump has said he wants a “quick” deal after that happens and the two leaders are expected to discuss future co-operation.
Ted Malloch, who has been tipped as a future US ambassador to the European Union, has told the BBC the outline of a free trade deal could be done in as little as 90 days.
But other economists have warned that although the UK and US are already significant trade partners, negotiating a bilateral agreement from scratch could take years and face many potential obstacles.
Mr Trump’s protectionist rhetoric in his inauguration speech and his apparent hostility to existing global trade deals – he has already withdrawn from the Trans-Pacific Partnership and urged major reform of the North American Free Trade Agreement (Nafta) – have raised question marks about his commitment to an equal partnership.
Mr Trump is expected to make a brief appearance at the three-day Republican retreat in Philadelphia, an annual strategy event held behind closed doors.
Although the party also controls the House of Representatives and the Senate, Mr Trump fell out with many leading party figures during the presidential campaign.