Israel's Supreme Court has upheld a government decision to order a senior Human Rights Watch employee to leave.
The interior ministry said it had revoked the work permit of US citizen Omar Shakir in 2018 because he had supported a boycott of Israel.
It based the decision on a dossier covering his activities over the previous 10 years, almost all of them predating his Human Rights Watch role.
The group insists that neither it nor Mr Shakir promote boycotts of Israel.
Following Tuesday's court ruling, Mr Shakir tweeted that the decision on whether to deport him "now shifts back" to the Israeli government.
"If it proceeds, I have 20 days to leave & it'll join ranks of Iran, N Korea & Egypt in blocking access for @hrw official. We won't stop. And we won't be the last."
Israeli Interior Minister Arye Deri welcomed the ruling, saying: "Anyone who works against the state should know that we will not allow him to live or work here".
The interior ministry argued that Mr Shakir was an "activist" for the Boycott, Divestment and Sanctions (BDS) movement, which campaigns for a complete boycott of Israel over its policies towards the Palestinians.
Israel says that BDS opposes the country's very existence and is motivated by anti-Semitism. In 2017, it passed a law refusing entry to people with links to BDS.
Human Rights Watch (HRW) rejected the Israeli government's portrayal of Mr Shakir.
It stressed that as its representative he had called on companies to stop working in or with Jewish settlements in the occupied West Bank, and had not called for a consumer boycott of those companies. The settlements are considered illegal under international law, though Israel disputes this.
HRW also argued that the 2017 law violated constitutionally protected fundamental liberties, including freedom of expression and the prohibition of discrimination based on political or ideological convictions.
Former Israeli officials and human rights groups filed motions to join Mr Shakir's appeal, while the European Union and United Nations Secretary General António Guterres called on Israel not to deport him.