British authorities have announced that 23 Russian diplomatic staff are to be expelled from the UK as a result of the Salisbury nerve attack - with Moscow announcing that it will take retaliatory measures in response.
A political expert and former career diplomat explains to ITV News how it works - and what happens to those who are ordered to leave.
- How can a state expel another country's diplomats?
Most embassy staff abroad enjoy diplomatic immunity from prosecution in their host country - but their presence depends on permission that can be withdrawn at any moment.
Article nine of the Vienna convention says the host country can declare any member of foreign diplomatic staff as "persona non grata" and require them to be removed.
This can be done by a state "at any time and without having to explain its decision", it adds.
- How do embassy workers find out they're being ordered out?
The ambassador is usually summoned by the host country and given a list of all those being ordered out - which might include themselves.
It's then up to them to go and break the news to their staff.
- How long do staff get to leave? Can they refuse?
Usually expelled staff are given just days to leave the country, or sometimes as little as 24 hours, says James Hoare, an associate fellow at Chatham House and former career diplomat.
They will be rushed out on flights, often leaving many of their possessions they brought to last them through the years-long postings behind.
It will then be left to other embassy workers to pack up their property and send it on for them.
Refusing to cooperate with an order from a host country is almost always nonviable and would draw an immediate response, said Mr Hoare.
"You would open up a diplomatic war which would not allow any of your people to function," he said.
In similar cases, police and security forces have surrounded an embassy, preventing all staff from entering or leaving and effectively halting all its diplomatic work until the standoff is resolved.
- Why could diplomatic staff be expelled?
For many of those who are expelled, they are simply in the wrong place at the wrong time and are caught up in an international row.
Mr Hoare says anyone identified as a covert intelligence operative working under a cover story is likely to be ordered out.
But research analysts or country experts are also frequently told to leave.
"If you want to go for the maximum impact, you go for the people who you think know most about your country," said Mr Hoare.
The aim would be to reduce your rival's ability to understand your country's political system and actions, and hamper their work within the country, he said.
In such cases, there are "almost invariably" retaliations from the affected country with similar expulsions of diplomatic staff there.
There are also cases in which a member of diplomatic staff is expelled as a result of breaking the law or personally carrying out an inflammatory act. Diplomatic immunity means they cannot be prosecuted, but they can be ordered to leave.
- Can they ever return?
For many of those blacklisted and ordered out, there is no way they can ever set foot in the country again.
"If a person is declared persona non grata, it is quite likely they won't be allowed back in," said Mr Hoare.
For people who have dedicated much of their life to learning a foreign language and political systems of a country it can be devastating and a huge career blow.
- What are the political after-effects?
In contrast to the personal effect on those expelled, the countries involved in an expulsion can bounce back surprisingly quickly, said Mr Hoare.
He said that in the case of the latest Russian freeze in relations, a thaw may come "rather faster than you might think".
Much depends on whether the three victims survive and how long the case dominates headlines.
But practical considerations may push the UK to return to normal relations in the near future, he added.
"We probably need Russian trade; we do need Russian gas...You might find that we want a rapprochement with Russia rather sooner than later."