How are MPs deselected?
Deselection is the buzzword sending a shiver down the spine of MPs in both main parties as local activists, fired up over Brexit, flex their muscles.
Six of the nine former Labour MPs who quit the party, unhappy with Jeremy Corbyn's leadership and the party's handling of anti-Semitism, were previously facing local pressure to remove them as MPs.
And the three former Conservative MPs - Anna Soubry, Sarah Wollaston and Heidi Allen - who quit to join the Independent Group, claim a "purple wave" of former UKIP members are joining local Tory associations purely to deselect Remain supporting MPs.
Local parties select candidates for general elections (sometimes with a little help from party HQ) - and they can sack an MP once they have been elected, ensuring that they can't stand under the party banner in that area at the next general election.
Local activists argue that MPs do not have the right to a job for life - and that they should be open to a challenge if they no longer reflect the views of party members or their electorate.
MPs, meanwhile, insist they are entitled to express their opinions without being hounded out of Parliament.
Deselection is not a straightforward process, particularly in the Labour Party, and MPs have several weapons at their disposal to fight it.
There are not currently thought to be any SNP or Lib Dem MPs facing the threat of deselection.
Four members of the Independent Group - Joan Ryan, Chris Leslie, Angela Smith and Gavin Shuker - had lost votes of no confidence in recent months, in their constituency, before quitting the Labour Party.
Such votes have no legal force in the party - they are merely a way of allowing activists to express their discontent, in effect putting the MP on notice of deselection.
After her confidence vote, Joan Ryan, a former minister under Tony Blair, said: "It never occurred to me that Trots, Stalinists, communists and assorted hard left would [have] confidence in me. I have none in them."
Luciana Berger had also been facing a no confidence motion but it was withdrawn amid accusations - denied by her Liverpool Wavertree party - that she was being "bullied".
Chuka Umunna had also been under pressure from activists in his Streatham constituency.
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The nine local Labour branches that have seen their MP quit the party will now have to select a new candidate for the next election, something they will presumably want to do as soon as possible.
Who else is under threat in Labour?
But it is not just the Labour defectors who have been feeling the heat: some reports claim up to 100 Labour MPs could be challenged as the Labour candidate before the next election.
Staunch Jeremy Corbyn loyalist Chris Williamson, who last year toured the country with a "democracy roadshow" calling for the mandatory reselection of every Labour MP, has been threatened with deselection by members of his Derby North constituency party.
Veteran Labour Brexiteer Kate Hoey has vowed to fight deselection after losing a confidence vote in her Vauxhall constituency, over her stance on the EU and other issues.
There were fears among some in the party that Labour's recent "democracy review", which recommends changes to the party's internal structure, would advocate the mandatory reselection of every MP but instead it pushed back any decision until after the proposed constituency boundary review.
In their resignation letter, the three Conservative defectors said their former party is being subsumed by a UKIP-inspired "purple momentum" shifting the party to the right, particularly over Brexit.
Broxtowe MP Anna Soubry said she knew of certain MPs who have been "frightened", unable to vote as they wish for fear of deselection or being admonished by their local association.
Before quitting, Dr Wollaston and Ms Allen, who both advocate a second referendum, were facing pressure in their local parties over their views, with crunch meetings planned in the next few weeks.
Who else is under threat in the Conservatives?
Of those who remain in the Conservative Party, a process that may lead to the deselection of former minister Nick Boles, who is behind attempts to prevent a no-deal Brexit, is under way in his Grantham and Stamford constituency.
His constituency party chairman, Philip Sagar, has already said his local members want him out and he "must go now". But Mr Boles has vowed to fight any attempts to oust him.
A petition calling for the Remain-supporting MP Dominic Grieve to be deselected by his local Beaconsfield Conservative Association has attracted more than 35,000 signatures.
And Sir Alan Duncan is reported by the Telegraph to be facing a no-confidence vote organised by Eurosceptic activists, at the annual meeting of his Melton and Mowbray association.
Displeasure has even spread to sections of the parliamentary party. Conservative MP Nadine Dorries has called for some of her more rebellious colleagues, such as Dominic Grieve, to be deselected.
Deselection is not a recent phenomenon. In 1904, even the now-heralded wartime Prime Minister Sir Winston Churchill faced the ignominy of being deselected as the local Conservative candidate, by his Oldham association, in a row over his support for free trade.
He continued to sit as the town's MP but joined the Liberal Party a few months later, for whom he won a Manchester seat in the 1906 election.
The most recent deselections have come from the Conservative side. Despite serving more than 30 years as member for South Suffolk, in 2015 Tim Yeo was unceremoniously rejected as the Conservative candidate by local members before the general election.
Days earlier, Thirsk and Malton MP Anne McIntosh, who now sits in the House of Lords, suffered a similar fate.
Mr Yeo and Mrs McIntosh had both been prominent select committee chairs.
In Labour, actual deselections using the trigger ballot system are rare.
Former East Lothain MP Anne Moffatt was deselected in 2010, while in 2007 the MP for Liverpool West Derby, Bob Wareing, blamed a "New Labour mafia" for his deselection in favour of Stephen Twigg.
How to remove an MP
The two main UK parties have different rules.
In the run-up to a general election, sitting Labour MPs can be made to compete for selection as a candidate against all-comers.
But until recently, they faced such a reselection contest only if 50% of a constituency's local branches and affiliated unions voted for it in a "trigger ballot".
At its conference last September, however, the party made it easier to deselect MPs - by cutting this threshold to 33%.
In the Conservative Party, the process is simpler.
The sitting MP must apply in writing to be readopted as the party candidate.
A specially convened meeting follows, where the executive council votes on the MP's future.
Additionally, if a petition gathers signatures from 50 local members (or 10% of total membership) this can also force a meeting.
If the MP loses the vote, they must take part in a new candidate selection.