An Oxford University professor had to hold an academic conference in private for fear of disruption from activists, it has emerged.

Earlier this year, Professor Nigel Biggar hosted a summit of historians, economists and ethicists to discuss the legacy of colonialism, but the invitation-only event took place behind closed doors.

The event was aimed at reflecting on the American political science professor Bruce GiIley's controversial article, titled "The Case for Colonialism", which argues that colonial rule was sometimes legitimate and that post-colonial states would do better to nurture certain aspects of their colonial legacy than discard them.

Prof Biggar said that he often holds such meetings in private because "the quality of discussion is better with a small, hand-picked group". He said: "But on this occasion [it was] also because I didn't want to risk activist disruption."

He explained that another reason for holding the event in private was that one young scholar would only attend on condition of anonymity, “lest his attendance come to the attention of some of his senior colleagues and “count it against his career”.

Prof Biggar, who is head of the McDonald Centre for Theology, Ethics, and Public Life, is currently leading a project on "Ethics and the Empire", which analyses the impact of Britain's imperial past.