Until July 2019, Gilligan was senior correspondent of The Sunday Times and had also served as head of the Capital City Foundation at Policy Exchange.
[2][3][4] He is best known for a 2003 report on BBC Radio 4's Today programme in which he described a British government briefing paper on Iraq and weapons of mass destruction (the September Dossier) as having been "transformed in the week before it was published to make it sexier".
[11] In May 2003, Gilligan made a broadcast in which he claimed that the British Government had "sexed up" a report in order to exaggerate the weapon of mass destruction capabilities of Saddam Hussein.
[5] He was also a reporter for Channel 4's investigative programme Dispatches, covering a number of issues, including Rahman and his claimed involvement with the Islamic Forum of Europe in the London borough of Tower Hamlets.
[30] He was reportedly instrumental in persuading Sunak to abandon the proposed northern leg of the High Speed 2 railway infrastructure project.
[31][32][33] In May 2016, The Telegraph apologised and paid substantial damages as part of an out-of-court settlement for defamation due to false claims made by Gilligan in a series of articles alleging corruption surrounding the purchase of Poplar Town Hall by businessman Mujibul Islam from Tower Hamlets Council when Rahman was mayor.
[34] In May 2018, The Sunday Telegraph paid "substantial damages" to settle a defamation case brought against it by Mohammed Kozbar in relation to an article by Gilligan in March 2016.
[36] In January 2019, The Sunday Times was required to publish a correction by the Independent Press Standards Organisation, which ruled that an article in July 2018 by Gilligan about laws regarding transgender people had been "misleading".