diff --git a/app/models/applicants/personal_detail.rb b/app/models/applicants/personal_detail.rb
index 2f381a3d..418abf59 100644
--- a/app/models/applicants/personal_detail.rb
+++ b/app/models/applicants/personal_detail.rb
@@ -19,7 +19,8 @@ class PersonalDetail
     validate :age_less_than_maximum
     validate :minimum_age
     validates :sex, presence: true, inclusion: { in: SEX_OPTIONS }
-    validates :passport_number, presence: true
+    validates :passport_number, presence: true, length: { maximum: 20, message: "passport number is invalid" }
+    validate :valid_passport_number
     validates :nationality, presence: true, inclusion: { in: NATIONALITIES }
     validates :address_line_1, presence: true
     validates :city, presence: true
@@ -91,6 +92,21 @@ def age_less_than_maximum
       errors.add(:date_of_birth)
     end
 
+    def valid_passport_number
+      return if passport_number.blank?
+
+      # Reject if it contains any characters other than alphanumeric
+      unless passport_number =~ /\A[a-zA-Z0-9]+\z/
+        errors.add(:passport_number, 'passport number is invalid')
+        return
+      end
+
+      # Reject if it doesn't contain at least one number
+      unless passport_number =~ /\d/
+        errors.add(:passport_number, 'passport number is invalid')
+      end
+    end
+
     def minimum_age
       # rubocop:disable Rails/Blank
       return unless date_of_birth.present?
diff --git a/spec/models/applicants/personal_detail_spec.rb b/spec/models/applicants/personal_detail_spec.rb
index 14d719b0..d8621985 100644
--- a/spec/models/applicants/personal_detail_spec.rb
+++ b/spec/models/applicants/personal_detail_spec.rb
@@ -190,6 +190,36 @@ module Applicants
           end
         end
       end
+
+      describe "passport_number validations" do
+        subject(:model) { described_class.new(passport_number:) }
+
+        before { model.valid? }
+
+        context "when passport_number is valid" do
+          valid_passports = %w[PAT34566 A345667 JS3445 434455AFT 2453454]
+
+          valid_passports.each do |valid_passport|
+            let(:passport_number) { valid_passport }
+
+            it "#{valid_passport} should be valid" do
+              expect(model.errors.messages_for(:passport_number)).to be_blank
+            end
+          end
+        end
+
+        context "when passport_number is invalid" do
+          invalid_passports = ["asdfasdf", "nil", "%$^%%^%"]
+
+          invalid_passports.each do |invalid_passport|
+            let(:passport_number) { invalid_passport }
+
+            it "#{invalid_passport} should be invalid" do
+              expect(model.errors.messages_for(:passport_number)).to include("passport number is invalid")
+            end
+          end
+        end
+      end
     end
   end
 end