Select three referees with seniority, independence, and field knowledge
Three referees are required. Each must be an established researcher — typically professorial level or a senior figure in a research institution — who can speak independently to the quality of your work. At least two should be from institutions other than your current employer; ideally at least one should be internationally based.
Referee seniority matters more in academic applications than in tech applications. A letter from a Nobel laureate or FRS carries significant weight; a letter from a junior lecturer, however well-intentioned, may actually weaken the case by implying your strongest advocates are not senior. Approach referees early — academic letter-writing takes time, and a rushed letter is evident. Provide a clear briefing: your chosen tier, your three or four most significant contributions, and the specific sub-criteria you're asking them to address.
Independence is assessed carefully. A referee who is your doctoral supervisor, your spouse's colleague, or a co-author on more than three recent papers may be viewed as too close to your work. This doesn't disqualify them, but the panel will weight their letter accordingly. A letter from someone who knows your work primarily through the literature — who has cited you, reviewed your grants, or referenced your datasets — can be more persuasive than one from a close collaborator, precisely because it demonstrates that your influence has propagated beyond your immediate network.
Give your referees a structured briefing document that includes: the endorsing body and tier you're applying under, the specific criteria, your two or three most significant contributions with DOIs or outputs, the word count target for the letter (400–700 words is typical), and a deadline that gives them 5–6 weeks. Academic referees write many letters; the ones that are most useful to your application are the ones where the referee has enough context to write specifically, not generically.