LONDON — Threats of sexual violence. Death threats. Racist abuse.

Female members of Britain’s Parliament have long said that this type of vitriolic abuse, both online and off, is increasingly common. Now, several women who have decided not to run for re-election when British voters go to the polls next month say that abuse, threats and a culture of intimidation are a significant reason they are leaving Parliament.

“I am exhausted by the invasion into my privacy and the nastiness and intimidation that has become commonplace,” Heidi Allen, a member of Parliament, wrote in a detailed letter to her constituents explaining why she will not be on the ballot. “Nobody in any job should have to put up with threats, aggressive emails, being shouted at in the street, sworn at on social media, nor have to install panic alarms at home.”

Ms. Allen is among more than 50 members of Parliament, including 18 women, who have said they will not stand in the Dec. 12 election. And while that ratio roughly mirrors Parliament’s overall gender breakdown, women’s rights activists worry that this climate could deter other women from entering national politics, reversing a trend that had brought the proportion of female lawmakers in the House of Commons to an all-time high of 32 percent .

Rights groups also say that an increasingly bitter political climate surrounding Brexit has fueled the flames — as did recent remarks by Prime Minister Boris Johnson on the treatment of female lawmakers.