Uptake of colorectal cancer screening is suboptimal. The TEMPO trial evaluated the impact of two evidence-based, theory-informed, and co-designed behavioural interventions on uptake of faecal immunochemical test (FIT) colorectal screening.TEMPO was a 2 × 4 factorial, eight-arm, randomised controlled trial embedded in the nationwide Scottish Bowel Screening Programme. All 40 000 consecutive adults (aged 50-74 years) eligible for colorectal screening were allocated to one of eight groups using block randomisation: (1) standard invitation; (2) 1-week suggested FIT return deadline; (3) 2-week deadline; (4) 4-week deadline; (5) problem-solving planning tool (no deadline); (6) planning tool plus 1-week deadline; (7) planning tool plus 2-week deadline; (8) planning tool plus 4-week deadline. The primary outcome was the proportion of FITs returned correctly completed to be tested by the colorectal screening laboratory providing a positive or negative result, within 3 months of the FIT being mailed to a person. The trial is registered with clinicaltrials.gov, NCT05408169.From June 19 to July 3, 2022, 5000 participants were randomly assigned per group, with no loss to follow-up. 266 participants met the exclusion criteria; 39 734 (19 909 [50·1%] female and 19 825 [49·9%] male; mean age 61·2 [SD 7·3] years) were included in the analysis. The control group (no deadline, and no planning tool) had a 3-month FIT return rate of 66·0% (3275 of 4965). The highest return rate was seen with a 2-week deadline without the planning tool (3376 [68·0%] of 4964; difference vs control of 2·0% [95% CI 0·2 to 3·9]). The lowest return rate was seen when the planning tool was given without a deadline (3134 [63·2%] of 4958; difference vs control of -2·8% [-4·7 to -0·8]). The primary analysis, assuming independent effects of the two interventions, suggested a clear positive effect of giving a deadline (adjusted odds ratio [aOR] 1·13 [1·08 to 1·19]; p<0·0001), and no effect for use of a planning tool (aOR 0·98 [0·94 to 1·02]; p=0·34), though this was complicated by an interaction between the two interventions (pinteraction=0·0041); among those who were given a deadline, there was no evidence that receiving a planning tool had any effect (aOR 1·02 [0·97 to 1·07]; p=0·53), but in the absence of a deadline, giving the planning tool appeared detrimental (aOR 0·88 [0·81 to 0·96]; p=0·0030). In the absence of the planning tool, there was little evidence that the use of a deadline had any effect on return rates at 3 months. However, secondary analyses indicated that the use of deadlines boosted earlier return rates (within 1, 2, and 4 weeks, particularly around the time of the deadline), and reduced the need to issue a reminder letter after 6 weeks, with no evidence that the planning tool had any positive impact, and without evidence of interactions between interventions.Adding a single sentence suggesting a deadline for FIT return in the invitation letter to FIT colorectal screening resulted in more timely FIT return and reduced the need to issue reminder letters. This is a highly cost-effective intervention that could be easily implemented in routine practice. A planning tool had no positive effect on FIT return.Scottish Government and Cancer Research UK.