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.

Adrian Ajao sent a final message via WhatsApp three minutes before he launched his terrorist rampage, and Scotland Yard confirmed yesterday 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 Ajao. 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.