In the test case that came before the Supreme Court in 2010, Holder v. Humanitarian Law Project, the Court held that it was constitutional to prohibit a group of humanitarian legal
professionals (including a retired U.S. judge) «from engaging in
certain specified activities, including training PKK members to use international law to resolve disputes peacefully; teaching PKK members to petition the United Nations and
other representative
bodies for relief; and engaging in political advocacy on behalf of Kurds living in Turkey and Tamils living in Sri Lanka.»
Under English law «legal
professional privilege» entitles a party to withhold evidence (electronic, written, or oral) and not disclose it to either the
other side, the court, or regulatory
bodies (subject to
certain exceptions).