RSF Secretary-General Christophe Deloire and Director of Operations Rebecca Vincent were not able to carry out their visit with Assange, a visit which had taken months to secure. Belmarsh Prison Governor Jenny Louis denied their visit at the last minute because she had "received intelligence" that the two were journalists.

"We followed all of the prison's procedures and rules. We arrived early with all documents. But we were told at reception that we would not be allowed in," tweeted Vincent.

The RSF leaders have made it clear that they are not in fact journalists. "[RSF] defends journalists. Some of my colleagues have journalistic backgrounds. But we work as an NGO to support journalists," wrote Vincent. "Our prison visit would be as an NGO, not journalistic."

April 5, will mark 13 years since WikiLeaks published the infamous "Collateral Murder" video, which showed the US military killing of 12 civilians including two Reuters journalists in Baghdad.

"What has happened this morning is a shameful act and it is actually part of a pattern," said Assange's wife Stella outside of the prison. She was allowed her visit with Assange while Deloire and Vincent were barred. "This is a political persecution against a publisher who was only doing his job."