Primary care is an important setting for smoking cessation interventions. There is evidence for the effectiveness of tailored interventions for smoking cessation, and text messaging interventions for smoking cessation show promise. The intervention to be evaluated in this trial consists of two components: (1) a web-based program designed to be used by a practice nurse or other smoking cessation advisor (SCA); the program generates a cessation advice report that is highly tailored to relevant characteristics of the smoker; and (2) a three-month programme of automated tailored text messages sent to the smokers mobile phone. The objectives of the trial are to assess the acceptability and feasibility of the intervention and to estimate the short-term effectiveness of the intervention in increasing the quit rate compared with usual care alone. The design is a two parallel group randomised controlled trial (RCT). 600 smokers who want to quit will be recruited in up to 30 general practices in the East of England. During a consultation with an SCA, they will be individually randomised by computer program to usual care (Control) or to usual care plus the iQuit system (Intervention). At the four-week follow-up appointment, the SCA will record smoking status and measure carbon monoxide level. There will be two further follow-ups, at eight weeks and six months from randomisation date, by postal questionnaire sent from and returned to the study centre or by telephone interview conducted by a research interviewer. The primary outcome will be self-reported abstinence for at least two weeks at eight weeks. A sample size of 300 per group would give 80% power to detect an increase in quit rate from 20% to 30% (alpha = 0.05, 2-sided test). The main analyses of quit rates will be conducted on an intention-to-treat basis, making the usual assumption that participants lost to follow up are smoking. This trial will focus on acceptability, feasibility and short-term effectiveness. The findings will be used to refine the intervention and to inform the decision to proceed to a pragmatic trial to estimate longer-term effectiveness and cost-effectiveness. ISRCTN56702353.