Inclusion criteria:

          -  Provision of written informed consent prior to study entry

          -  Female participants aged at least 18 years

          -  Post-menopausal status defined as meeting at least one of the following criteria:

               1. Have undergone a bilateral oophorectomy

               2. Age ≥ 60 years

               3. Age ≥ 50 and < 60 years and with cessation of menses ≥ 12 months and
                  follicle-stimulating hormone and oestradiol levels in the post-menopausal range
                  and with an intact uterus in the absence of oral contraception or hormone
                  replacement therapy prior to the diagnosis of breast cancer

          -  Pre-menopausal status (Stage 2 only) defined as participants who are naturally
             pre-menopausal (i.e. without concurrent ovarian suppression)

             (a) Women of childbearing potential must: (i) Not be breastfeeding (ii) Have a
             negative pregnancy test prior to the start of dosing (iii) Agree to use one highly
             effective barrier method of contraception from the time of screening until 4 weeks
             after discontinuing study treatment

          -  Female participants with newly diagnosed primary breast cancer scheduled to undergo
             treatment with curative intent by surgery and irrespective of clinical node status

          -  Histologically confirmed invasive breast cancer involving a palpable tumour of any
             size, or a tumour with an ultrasound assessed diameter of ≥ 1.0 cm

          -  Participants with adequately treated non-melanoma skin cancer, curatively treated
             in-situ cancer of the cervix, or other solid tumours curatively treated with no
             evidence of disease for ≥ 3 months can be considered for the study

          -  According to the local laboratory participants must have:

               1. ER positive breast cancer

               2. HER2-negative breast cancer

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0 to 1

        Exclusion Criteria:

          -  Previous systemic or local treatment for the new primary breast cancer currently under
             investigation (including surgery, radiotherapy, cytotoxic and endocrine treatments)

          -  Intervention with any of the following:

               1. Use of sex-hormone-containing drugs within 6 months prior to the first dose of
                  study treatment

               2. Medications or herbal supplements known to be strong inhibitors/inducers of
                  CYP3A4/5, sensitive CYP2B6 substrates and drugs which are substrates of CYP2C9
                  and/or CYP2C19 which have a narrow therapeutic index

               3. Drugs that are known to prolong QT and have a known risk of torsades de pointes

          -  Inflammatory breast cancer

          -  Any evidence of severe or uncontrolled systemic diseases which in the investigator's
             opinion makes it undesirable for the participant to participate in the study

          -  Any of the following cardiovascular criteria: Mean resting QTcF > 470 msec; resting
             heart rate of < 50 bpm; any clinically important abnormalities in rhythm, conduction,
             or morphology of resting ECG; any factors that increase the risk of QTc prolongation
             or risk of arrhythmic events; known left ventricular ejection fraction < 50%;
             significant cardiovascular procedure or event within the last 6 months; uncontrolled
             hypertension or symptomatic hypotension

          -  Inadequate bone marrow reserve or organ function

          -  Refractory nausea and vomiting, uncontrolled chronic gastrointestinal diseases,
             inability to swallow the formulated product, or previous significant bowel resection
             that would preclude adequate absorption of AZD9833

          -  History of hypersensitivity to active or inactive excipients of AZD9833, or
             fulvestrant (stage 2 only)

          -  Previous randomisation in the present study

          -  Judgement by the Investigator that the participant should not participate in the study
             if the participant is unlikely to comply with study procedures, restrictions and
             requirements