It is hypothesised that the number of donations made by English blood donors will be greater with reduced vs. standard inter-donation intervals. The null hypothesis is that there will be no difference in donations between treatment groups; this may arise if reduced inter-donation intervals result in a greater number of donation deferrals (due to low haemoglobin) and/or an unacceptable burden to donors.

50,000 blood donors will be recruited from permanent blood donation centres across England to compare different intervals between blood donations to try to find the optimum interval for which it is safe for different donors to give blood. The study will look at whether intervals should be tailored by age, gender, genetic profile, and other characteristics. Study findings should help to improve the well-being of future blood donors in England and enhance the country's blood supplies.

Inclusion Criteria:

1. Aged 18 years or older and fulfilling all normal criteria for blood donation
2. Willing to be assigned to any of the study intervention groups
3. Registered at one of the permanent donation clinics at the time of enrolment

Exclusion Criteria:

• As the aim of the study is to be almost "paper-less", it will involve remote web-based data collection. Hence, participants who do not have internet access and/or are not willing to provide an email address for study correspondence will be excluded.