diff --git a/README.md b/README.md
index 3e1e9fd..807b457 100644
--- a/README.md
+++ b/README.md
@@ -69,6 +69,7 @@ The following checks are performed when calling the binary:
 - `pkgs/by-name` must only contain subdirectories of the form `${shard}/${name}`, called _package directories_.
 - The `name`'s of package directories must be unique when lowercased.
 - `name` is a string only consisting of the ASCII characters `a-z`, `A-Z`, `0-9`, `-` or `_`.
+- `name` only starts with an ASCII character `a-z`, `A-Z`, `-` or `_`.
 - `shard` is the lowercased first two letters of `name`, expressed in Nix: `shard = toLower (substring 0 2 name)`.
 - Each package directory must contain a `package.nix` file and may contain arbitrary other files.
 
diff --git a/src/problem/mod.rs b/src/problem/mod.rs
index 981c0ea..0550903 100644
--- a/src/problem/mod.rs
+++ b/src/problem/mod.rs
@@ -34,6 +34,8 @@ pub mod npv_161;
 pub mod npv_162;
 pub mod npv_163;
 
+pub mod npv_170;
+
 #[derive(Clone, Display, EnumFrom)]
 pub enum Problem {
     /// NPV-100: attribute is not defined but it should be defined automatically
@@ -123,6 +125,9 @@ pub enum Problem {
     NewTopLevelPackageShouldBeByNameWithCustomArgument(
         npv_163::NewTopLevelPackageShouldBeByNameWithCustomArgument,
     ),
+
+    /// NPV-170: by-name package cannot be number prefixed
+    ByNamePackegPrefixedWithNumber(npv_170::ByNamePackegPrefixedWithNumber),
 }
 
 fn indent_definition(column: usize, definition: &str) -> String {
diff --git a/src/problem/npv_170.rs b/src/problem/npv_170.rs
new file mode 100644
index 0000000..c540051
--- /dev/null
+++ b/src/problem/npv_170.rs
@@ -0,0 +1,25 @@
+use std::fmt;
+
+use derive_new::new;
+use relative_path::RelativePathBuf;
+
+#[derive(Clone, new)]
+pub struct ByNamePackegPrefixedWithNumber {
+    #[new(into)]
+    package_name: String,
+    #[new(into)]
+    relative_package_dir: RelativePathBuf,
+}
+
+impl fmt::Display for ByNamePackegPrefixedWithNumber {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let Self {
+            package_name,
+            relative_package_dir,
+        } = self;
+        write!(
+            f,
+            "- {relative_package_dir}: Attribute `{package_name}` should not be number-prefixed. Prefix with `_`, or wrap in quotes"
+        )
+    }
+}
diff --git a/src/structure.rs b/src/structure.rs
index 93b5975..966326d 100644
--- a/src/structure.rs
+++ b/src/structure.rs
@@ -7,7 +7,9 @@ use lazy_static::lazy_static;
 use regex::Regex;
 use relative_path::RelativePathBuf;
 
-use crate::problem::{npv_109, npv_110, npv_111, npv_140, npv_141, npv_142, npv_143, npv_144};
+use crate::problem::{
+    npv_109, npv_110, npv_111, npv_140, npv_141, npv_142, npv_143, npv_144, npv_170,
+};
 use crate::references;
 use crate::validation::{self, ResultIteratorExt, Validation::Success};
 use crate::NixFileStore;
@@ -16,8 +18,9 @@ pub const BASE_SUBPATH: &str = "pkgs/by-name";
 pub const PACKAGE_NIX_FILENAME: &str = "package.nix";
 
 lazy_static! {
-    static ref SHARD_NAME_REGEX: Regex = Regex::new(r"^[a-z0-9_-]{1,2}$").unwrap();
-    static ref PACKAGE_NAME_REGEX: Regex = Regex::new(r"^[a-zA-Z0-9_-]+$").unwrap();
+    static ref SHARD_NAME_REGEX: Regex = Regex::new(r"^((_[0-9])|([a-z][a-z0-9_-]?))$").unwrap();
+    static ref PACKAGE_NAME_REGEX: Regex =
+        Regex::new(r"^((_[0-9])|[a-zA-Z])[a-zA-Z0-9_-]*$").unwrap();
 }
 
 /// Deterministic file listing so that tests are reproducible.
@@ -137,11 +140,19 @@ fn check_package(
     } else {
         let package_name_valid = PACKAGE_NAME_REGEX.is_match(&package_name);
         let result = if !package_name_valid {
-            npv_141::InvalidPackageDirectoryName::new(
-                package_name.clone(),
-                relative_package_dir.clone(),
-            )
-            .into()
+            if package_name.starts_with(|c: char| c.is_ascii_digit()) {
+                npv_170::ByNamePackegPrefixedWithNumber::new(
+                    package_name.clone(),
+                    relative_package_dir.clone(),
+                )
+                .into()
+            } else {
+                npv_141::InvalidPackageDirectoryName::new(
+                    package_name.clone(),
+                    relative_package_dir.clone(),
+                )
+                .into()
+            }
         } else {
             Success(())
         };
diff --git a/tests/by-name-numprefix/default.nix b/tests/by-name-numprefix/default.nix
new file mode 100644
index 0000000..861260c
--- /dev/null
+++ b/tests/by-name-numprefix/default.nix
@@ -0,0 +1 @@
+import <test-nixpkgs> { root = ./.; }
diff --git a/tests/by-name-numprefix/expected b/tests/by-name-numprefix/expected
new file mode 100644
index 0000000..38342a6
--- /dev/null
+++ b/tests/by-name-numprefix/expected
@@ -0,0 +1,2 @@
+- pkgs/by-name/_1/10foo: Attribute `10foo` should not be number-prefixed. Prefix with `_`, or wrap in quotes
+This PR introduces the problems listed above. Please fix them before merging, otherwise the base branch would break.
diff --git a/tests/by-name-numprefix/pkgs/by-name/_1/10foo/package.nix b/tests/by-name-numprefix/pkgs/by-name/_1/10foo/package.nix
new file mode 100644
index 0000000..ce29e51
--- /dev/null
+++ b/tests/by-name-numprefix/pkgs/by-name/_1/10foo/package.nix
@@ -0,0 +1 @@
+{ someDrv }: SomeDrv