UPDATED: An executive order by President Trump signed on Friday 27th January, has halted the US refugee programme and banned anyone from seven countries - Iraq, Iran, Somalia, Yemen, Sudan, Syria and Libya - from travelling to the United States. It also suspended the Visa Waiver Interview Program.
Mr Trump's executive order restrictions also applies to dual citizens - so, for example, a British citizen who is also a citizen of one of the seven listed countries, will not be able to enter the US, even if they have already been approved for a visa, have an existing visa or a green card.
The Visa Waiver Interview Program, a waiver from having to come in for an interview to renew a current visa, not a waiver from having to obtain a visa in the first place, has also been suspended.
One high profile name affected by the ban is Sir Mo Farah. The star was born in Somalia, is a British citizen and trains in the US. On 20 January, seven days before Trump signed the executive order, Farah tweeted that he was in Ethiopia training for the London marathon. Sir Mo is no longer able to return to his training camp in the States.
Though interpretation of the policy is unclear, British nationals born in one of the seven countries with valid visas are been advised that they will be unable to enter into the US, even if they have family or relatives in the United States.
Mrs May initially said the USA's immigration policies were a matter for them, not her, before Downing Street said she "does not agree" with the entry ban.
For further information call the US Embassy in London 020 7499-9000 and follow their advice, ask if they have an updates email service, check back with them frequently, and hope that clarification comes sooner rather than later.
--- UPDATE 1: Originally this article referred to the Visa Waiver Program, it's the Visa Waiver Interview Program, which is a waiver from having to come in for an interview to renew a current visa, not a waiver from having to obtain a visa in the first place. The Visa Wavier program is by definition reciprocal —apologies for the error.
--- UPDATE 2: President Donald Trump's controversial executive order on refugees and immigration has been blocked by a federal judge. James Robart, a federal judge in Seattle ruled that the order be suspended temporarily “on a nationwide basis”, meaning government officials had to stop enforcing it and once-banned travellers could enter the country, on the grounds that the order was causing “immediate and irreparable injury” and may be ruled unconstitutional.
The state department reissued visas that had been invalidated, and airlines began to allow citizens of the seven countries to board planes to the US. The department of homeland security also said it would comply with the judge's order.