What UK laws can lead to arrest for tweets (e.g., communications offences, harassment, or terrorism laws)?
Executive summary
UK law allows arrests over social-media posts under several overlapping statutes: legacy “communications offences” (section 1 Malicious Communications Act 1988 and section 127 Communications Act 2003), the new communications offences in the Online Safety Act 2023, public order and harassment law, and terrorism offences — all cited by government, police and legal commentators as the legal basis for recent high‑profile arrests [1] [2] [3] [4]. Parliamentary debate and police chiefs say arrests for online messages have surged to “more than 30 a day” with over 12,000 arrests in 2023 reported by The Times, prompting calls to clarify thresholds between unlawful speech and free expression [5] [6] [1].
1. Communications offences: the old statutes that still matter
Two longstanding criminal provisions used to arrest people for tweets are section 1 of the Malicious Communications Act 1988 and section 127 of the Communications Act 2003; both criminalise sending indecent, grossly offensive or threatening messages and remain central to prosecutions of online posts [1] [7]. Law Commission work and government statements acknowledge these provisions are overlapping and ambiguous and will be reformed or replaced to improve clarity [8] [9].
2. The Online Safety Act: new “communications” crimes with higher penalties
Part 10 of the Online Safety Act 2023 introduced new offences — for example threatening communications, false communications, encouraging or assisting serious self‑harm and cyber‑flashing — that apply to electronic messages and can carry sentences up to five years in some cases [2] [10] [3]. Prosecutors and the CPS list the Act alongside the MCA 1988 and CA 2003 as the primary focus for communications offences [2].
3. Public order, harassment and civil remedies: multiple routes to arrest
Police and prosecutors routinely rely on public order law and the Protection from Harassment Act 1997 where a “course of conduct” causes alarm or distress; harassment needs at least two related acts but can lead to criminal charges and injunctions backed by arrest [11] [12] [13]. Defamation and civil claims also apply to tweets, and in some cases civil processes (libel, Norwich Pharmacal orders) interact with criminal policing [14] [15].
4. Terrorism law and “terrorist content” online
Material that meets the statutory definitions of terrorist content or conduct — including encouragement of terrorism, dissemination of terrorist publications or support for proscribed organisations — is criminal; government guidance treats online posting that constitutes such offences as reportable and prosecutable under terrorism legislation [4] [16] [17]. Counter‑terror policing and Ofcom’s duties under the Online Safety Act also press platforms to remove such material and report imminent threats [16] [18].
5. How policing practice and statistics shape the picture
Parliamentary debates and reporting indicate arrests for online communications rose sharply — more than 30 a day and a 121% increase from 2017 cited in Lords debate — yet convictions have not matched arrests, suggesting many cases end without prosecution or are resolved out of court [6] [1]. Critics argue this creates a chilling effect and that vague wording (e.g., “grossly offensive”, “annoyance” or “anxiety”) lets policing set de facto speech limits [5] [6].
6. Competing viewpoints: safety vs free expression
Police leaders (e.g., Metropolitan Police Commissioner) acknowledge officers acted under existing laws but have urged clearer thresholds so resources focus on threats causing real world harm; civil liberties groups and some MPs say vagueness drives over‑policing of speech and harms dissent [19] [20] [6]. Government documents frame Online Safety reforms as protecting people and enabling platform duties, while critics point to enforcement uncertainty and harms to lawful but offensive speech [9] [8] [21].
7. Practical takeaways for a tweeter in the UK
Available sources show the legal risks cover: messages judged “grossly offensive” or menacing under section 127/related MCA provisions; false or threatening communications and new Online Safety Act offences; harassment defined as a course of conduct under the Protection from Harassment Act 1997; and any post falling within terrorism offences [1] [2] [11] [4]. They also stress that police guidance, prosecutorial discretion and reform trajectories are in flux — the government and law‑reform bodies are working to replace or clarify older offences and implement the OSA’s new offences [9] [8] [22].
Limitations: sources summarised here are drawn from parliamentary debates, official CPS/Government guidance and reporting; they document statutes and enforcement trends but do not set out exhaustive charging thresholds or legal defence strategies — available sources do not mention specific line‑by‑line charging guidance for every factual scenario and legal advice should be sought in individual cases [2] [1].