(This story contains graphic language in the 19th paragraph)
By Kylie MacLellan and William James
LONDON (Reuters) - Death threats, a brick through a window and one lawmaker even installing a panic room in her office; less than a year after Britain's Labour elected socialist Jeremy Corbyn leader on a promise of 'kinder politics' the party is mired in civil war.
The brutal infighting threatens Labour's 116-year existence and risks leaving Britain without a strong opposition party for years to come, just as the government goes about negotiating the country's exit from the European Union.
Britain's June 23 EU referendum brought simmering tensions within Labour to the boil, resulting in a leadership challenge.
On one side are hundreds of thousands of vocal grassroots loyalists who are fiercely protective of Corbyn's left-wing principles. On the other are most Labour lawmakers, who say he did not do enough to prevent Brexit and is not a capable leader.
Corbyn now faces leadership rival Owen Smith in a contest so divisive it could ultimately lead to the party splitting if he is re-elected leader, as the bookmakers expect.
The debate has become so poisonous that most local party meetings have been suspended and Labour lawmakers say they have faced abuse ranging from rape threats to vandalized cars.
While most of the abuse happens online, the June killing of lawmaker Jo Cox on her way to a public meeting in her electoral district means colleagues are not taking any chances. Her murder is not however thought to be linked to Labour's internal woes.
"I'm upgrading my home security, my office security, there is going to be a panic room in my office. I have to carry around a thing that I can press so the police can find me," Labour lawmaker Jess Phillips told Reuters.
"I've had people putting my face on (a picture of) somebody with an arrow through their heart, a spear in their side. I've had people wishing that I was dead ... all to protect Corbyn."
Angela Eagle, who triggered the leadership contest by challenging Corbyn but has now pulled out to boost Smith's chances, has been advised by police not to hold open drop-in sessions with her constituents due to safety risks.
A brick was thrown through a window at her office in Wallasey, north west England, she has said, and police have arrested and bailed a 44-year-old man on suspicion of making threats to kill after an email was sent to the lawmaker.
Eagle, who is gay, has also faced homophobic slurs. Others have experienced anti-Semitism, and some have said members of Corbyn-supporting grassroots movement Momentum have been intimidating staff and constituents outside their offices.
A spokesman for Momentum said the party must settle its disputes democratically "without abuse, intimidation or coups".
CULTURE OF HATRED
Corbyn, a pacifist whose dissenting colleagues have described him as a "nice" and "decent" man, has condemned the abuse and urged members and supporters to "act with calm and treat each other with respect and dignity".
The 67-year-old says he has also received death threats.
But many of his lawmakers say they do not think he is not doing enough to match his words with action. Eagle told the Daily Telegraph newspaper that Corbyn had created a permissive environment where abuse is tolerated.
"Jeremy, this is being done in your name," 44 female Labour lawmakers wrote in a letter to Corbyn, asking him to do more to tackle the behavior of those involved. "The culture of hatred and division that is being sown does not benefit anybody."
Paula Sherriff, the lawmaker who organized the letter, told Reuters she had seen a significant increase in abuse since Corbyn's leadership was challenged, and the majority of it was coming from people who identified themselves as supporting him.
Sherriff said comments like those from finance spokesman John McDonnell to a rally in July that those plotting against his ally Corbyn were "fucking useless", were encouraging abuse.
"It is just so divisive and they need to take some responsibility for some of the behavior that has come as a result of that," she said. "Is there any wonder that some of the activists are going out and doing the same to us on Twitter?"
With Corbyn the favorite to win when the result is announced on Sept. 24, the party faces a struggle to re-unite.
Party donor Assem Allam is reported to have offered rebels funding to defect and form a new party or movement. While Labour lawmakers say they are focused on saving the party rather than splitting, they also acknowledge it will be difficult.
"There are massive wounds to heal ... Unless something is done, something directly happens, then I just can't see how I could stay," said Phillips.
Sherriff agrees, saying there will need to be mediation.
"I am not saying we can't come back from this and I desperately hope there isn't a split ... I want to try and bridge the divide that definitely exists. It will be really hard," she said.
LONG, HARD STRUGGLE
Corbyn has tapped into an appetite for change, winning support among disillusioned young voters and socialists who had drifted away from the party during two decades battling for the political center ground.
But critics say the leader, who has described Labour as a social movement, cannot win an election due in 2020. Some say his supporters are not even interested in doing so.
"Democracy gives power to people, 'Winning' is the small bit that matters to political elites who want to keep power themselves," Momentum Chair Jon Lansman said on Twitter in July.
Leadership contender Smith has warned that on its current path, the party risks being "consigned to history".
"I feel horrified that we are such a low ebb, horrified that we are where we are, that this party could be split," he told a rally in London last month.
"If we split, the forces of darkness, the forces of the radical right will flood into the gap that we leave. And in many parts of Britain we would be out of power for a long, long time."
Historical precedents for a new party don't bode well, said Steven Fielding, professor of political history at the University of Nottingham, predicting any split would only involve a small number of lawmakers rather than a wholesale walk out.
Most are more likely to bide their time and either wait until Corbyn loses an election, or launch another leadership bid when a rival has had more time to become established, he said.
An ICM poll last month gave Prime Minister Theresa May's Conservatives a 16-point lead over Labour, who on 27 percent support were at their lowest in that series since 2009.
"Labour has been divided for such a period that you just write off the next general election even under Owen Smith ... he has had the backing of the parliamentary Labour Party but then what will Momentum do?," said Fielding.
"In any scenario it is going to be a long, hard struggle, and before it gets better I think it will get worse for the Labour Party ... It is not a pretty prospect."
(Writing by Kylie MacLellan; editing by Giles Elgood)