More than 10,000 people have signed an e-petition on Downing Street's Web site against the pope's 4-day visit to England and Scotland in September, which will cost U.K. taxpayers an estimated £15 million ($22.5 million). The campaign has gained momentum as more Catholic sex abuse scandals have swept across Europe.
Although Benedict has not been accused of any crime, senior British lawyers are now examining whether the pope should have immunity as a head of state and whether he could be prosecuted under the principle of universal jurisdiction for an alleged systematic cover-up of sexual abuses by priests.
Universal jurisdiction - a concept in international law - allows judges to issue warrants for nearly any visitor accused of grievous crimes, no matter where they live. British judges have been more open to the concept than those in other countries.
Lawyers are divided over the immunity issue. Some argue that the Vatican isn't a true state, while others note the Vatican has national relations with about 170 countries, including Britain. The Vatican is also the only non-member to have permanent observer status at the U.N.
Then again, no other top religious leaders enjoy the same U.N. privileges or immunity, so why should the pope?
David Crane, former chief prosecutor at the Sierra Leone war crimes tribunal, said it would be difficult to implicate the pope in anything criminal.
"It's a fascinating kind of academic, theoretical discussion," said Crane, who prosecuted Sierra Leone's Charles Taylor when he was still a sitting head of state. "At this point, there's no liability at all."
But Geoffrey Robertson, who as a U.N. appeals judge delivered key decisions on the illegality of conscripting child soldiers and the invalidity of amnesties for war crimes, believes it could be time to challenge the immunity of the pope - and Britain could be the place. He wrote a legal opinion on the topic that was published Friday in the U.S. news site The Daily Beast and Saturday in the British newspaper the Guardian.
"Unlike in the United States, where the judges commonly uphold what the executive says, the British courts don't accept these things at face value," Robertson told The Associated Press on Saturday. "The Vatican is not a state - it was a construct of Italian dictator Benito Mussolini."
But Jeffrey Lena, the California attorney who argued - and won - head of state immunity for Benedict in U.S. sex abuse cases, said the pope could not successfully be prosecuted for crimes under international law.
"Those who would claim that 'universal jurisdiction' could be asserted over the pope appear to completely misunderstand the sorts of violations, such as genocide, which are required to assert such jurisdiction," he said in a statement to the AP.
Still, Israeli officials, including former Prime Minister Ehud Barak, have recently been targeted by groups in Britain under universal jurisdiction. The law principle is rooted in the belief that certain crimes - such as genocide, war crimes, torture and crimes against humanity - are so serious that they are an offense against humanity and must be addressed.
It's a tactic that the British government would likely abhor, but British judges have often gone against government wishes in lawsuits.
Recent examples include British judges who issued an arrest warrant against Israel's former foreign minister for alleged war crimes, and a British court ruling this year that forced the government to release its intelligence exchanges with U.S. officials about the torture claims of a former Guantanamo detainee.
Prosecution in the deepening cleric sex abuse scandal, however, ultimately rests on the question of immunity. If British judges do challenge the pope's immunity, there are a handful of possible legal scenarios - all of them speculative.
The pope could be served for a writ for civil damages, a complaint could be lodged with the International Criminal Court, or abuse victims could try to have Benedict arrested for crimes against humanity - perhaps the least-likely scenario.
Lawyers question whether an alleged systematic cover-up could be considered a crime against humanity - a charge usually reserved for the International Criminal Court - and whether it could be pursued under universal jurisdiction.
Attorney Jennifer Robinson in London, who has been researching the possibilities, says rape and sexual slavery can be considered crimes against humanity.
Others, like Hurst Hannum with the Fletcher School of Law & Diplomacy at Tufts University near Boston, are skeptical.
"No one would question that the Church's response to widespread abuses has been atrocious, but it's very difficult for me to see how that would fit 'crimes against humanity,'" said Hannum.
Robertson is more in favor of challenging the immunity question.
"Head of state immunity provides no protection in the International Criminal Court," said Robertson, who represented The Associated Press and other media organizations who sought to make U.S.-U.K. intelligence exchanges public in the case of former Guantanamo detainee Binyam Mohamed.
"If acts of sexual abuse by priests are not isolated or sporadic events but part of a wide practice both known to and unpunished by their de facto-authority - i.e. the Catholic Church ... then the commander can be held criminally liable," Robertson said.
Even though the Vatican - like the United States - did not sign the accord that established the international court, a crime would only have to occur in a country which did sign, like Britain. Still, lawyers would have to prove that the crimes or an alleged cover-up occurred or continued after the court was set up in July 2002.
In a 2005 test case in Texas that involved alleged victims of sex abuse by priests, the Vatican obtained the intervention of President George W. Bush, who agreed the pope should have immunity against such prosecutions because he was an acting head of a foreign state.
It was around 1929 when Mussolini decided that the Vatican - a tiny enclave about 0.17 of a square mile with some 900 people - was a sovereign state.
"The notion that statehood can be created by another country's unilateral declaration is risible," Robertson said.
Others say the last 80 years of history have turned the Vatican into a state, and it would be almost impossible to strip the pope of his immunity now.
"My guess is the weight of opinion would allow the pope to enjoy immunity," said Hannum. "It's not automatically clear that the Holy See is a state, although it's treated as one for almost every purpose."
Last year, a Palestinian bid to have Barak - the Israeli defense chief who also served as prime minister until 2001 - arrested for alleged war crimes during a visit to Britain failed when the courts determined that he should be given immunity from arrest.
But months later, pro-Palestinian activists persuaded a London judge to issue an arrest warrant for Israeli politician Tzipi Livni, who was foreign minister during the 2008-2009 war in Gaza. The warrant was eventually withdrawn after Livni canceled her trip.
Spain and Britain jointly pioneered the universal jurisdiction concept when, in 1998, Britain executed a Spanish arrest warrant for former Chilean dictator Augusto Pinochet on torture claims. Pinochet was kept under house arrest in London until he was ruled physically and mentally unfit to stand trial and released in 2000.
When he was arrested, however, Pinochet was no longer head of state.
In 2001, activists brought Israel's then-Prime Minister Ariel Sharon to trial in Belgium in connection with a 1982 massacre at a Beirut refugee camp. Sharon canceled a planned trip to Belgium and was tried in absentia in a Belgian court. He was not convicted but the case provoked diplomatic protests and prompted Belgium in 2003 to tighten the law that had permitted the trial.
Prime Minister Gordon Brown has vowed to block private groups from taking legal action against visiting foreign dignitaries but any new law is unlikely before Britain's expected May 6 election.
The pope plans to visit Malta, Portugal and Cyprus before traveling to Britain on Sept. 16. A trip to Spain is planned for later in the fall.
By Associated Press Writer Paisley Dodds; AP Writers Gregory Katz and Raphael Satter in London, Nicole Winfield in Rome and Daniel Woolls from Madrid contributed to this report