Donald Trump is under pressure to force WhatsApp to hand over vital evidence on the Westminster attack after Amber Rudd gained powerful allies in Europe in her fight to bring social media firms to heel.
Khalid Masood sent a final message via WhatsApp three minutes before he launched his terrorist rampage, and Scotland Yard confirmed on Monday that "[his] communications that day are a main line of enquiry".
Yet the US-based messaging company, which is owned by Facebook, has failed to hand over the contents of the communication, infuriating Amber Rudd, the Home Secretary.
Britain has no legal power to force WhatsApp to help investigators because it is based in America, but the US president could threaten the firm with legal action, as the FBI did in a similar fight with Apple last year.
An American tourist, Kurt Cochran, was one of the four people killed by Masood. Tourists from France, Germany, Poland, Italy, Greece, Ireland and Romania were among those injured.
Last night two members of the House of Commons Foreign Affairs committee urged Washington to help pressure WhatsApp executives to allow UK law enforcement officers to access the message.
Daniel Kawczynski, one Tory committee member of the committee, said: "There can be no question whatsoever for WhatsApp communication between terrorist and others to be kept secret, particularly, if police feel it could lead to further prosecutions
"Americans must put pressure on WhatsApp to comply otherwise we are entering uncharted waters."
Nadhim Zahawi, another Tory member of the committee, said that in this case alone the US authorities should urge WhatsApp to help further.
He said: "I think they should. And I would urge the executives at WhatsApp, it is so clear-cut that he is a criminal.
"It is like proceeds of crime. We would do the same for them. You surrender the right to privacy when you start killing innocent people."
Scotland Yard and the security services cannot access what could be a vital clue in their investigation because WhatsApp uses so-called "end-to-end encryption" which the firm says prevents even its own technicians from reading people's messages.
Ms Rudd has summoned WhatsApp, Facebook, Google and a host of other online companies to talks at the Home Office on Thursday and to urge them to stop extremists from "using social media as a platform".
Ministers stressed that the "UK Government and the US Government are as one" on encouraging American technology firms to co-operate.
One minister told The Daily Telegraph: "We both want to see these companies complying with requests for these things to be done and we want to see that. It is a challenge we all face."
Technology companies had to "wake up and realise they have a social responsibility [to help]," he said. "Being able to access this is how we catch paedophiles and how we catch terrorists."
Ms Rudd travelled to Brussels on Monday for a meeting of the EU Justice and Home Affairs Council.
Sources said that France and Estonia were among the countries to speak in support of Ms Rudd's assertion that online platforms such as WhatsApp should not be "a safe place for terrorists to communicate".
Ms Rudd said afterwards: "The threat we face is an international one. And preventing these attacks is a common aim."
It came as Microsoft warned Britain that it will not hand over messages that could help foil terrorist plots unless it is legally compelled to do so.
Microsoft said that it will not allow governments to "hack or attack any company anywhere" and when ministers go too far in their demands it will say no.
Lord Carlile, the former independent reviewer of terrorism legislation, said encryption poses a considerable problem for security services.
He said: "It deprives investigators of the metadata and content of communications which may be for a terrorist purpose."
The peer said he hoped new legislation would not needed in relation to the issue.
"I would expect co-operation from WhatsApp," he said. "However, if the service providers do not co-operate, legislation may have to be considered."