diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0bcf36c0af3..c0038417bb7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@
       - FIXED: Fixed Boost link flags in pkg-config file. [#6083](https://github.com/Project-OSRM/osrm-backend/pull/6083)
     - Routing:
       - FIXED: Fix generation of inefficient MLD partitions [#6084](https://github.com/Project-OSRM/osrm-backend/pull/6084)
+      - FIXED: Fix MLD level mask generation to support 64-bit masks. [#6123](https://github.com/Project-OSRM/osrm-backend/pull/6123)
 
 # 5.25.0
   - Changes from 5.24.0
diff --git a/include/partitioner/multi_level_partition.hpp b/include/partitioner/multi_level_partition.hpp
index 855aa5ae2f0..d83f9a341aa 100644
--- a/include/partitioner/multi_level_partition.hpp
+++ b/include/partitioner/multi_level_partition.hpp
@@ -186,11 +186,12 @@ template <storage::Ownership Ownership> class MultiLevelPartitionImpl final
             auto bits = static_cast<std::uint64_t>(std::ceil(std::log2(num_cells + 1)));
             offsets[lidx++] = sum_bits;
             sum_bits += bits;
-            if (sum_bits > 64)
+            if (sum_bits > NUM_PARTITION_BITS)
             {
                 throw util::exception(
                     "Can't pack the partition information at level " + std::to_string(lidx) +
-                    " into a 64bit integer. Would require " + std::to_string(sum_bits) + " bits.");
+                    " into a " + std::to_string(NUM_PARTITION_BITS) +
+                    "bit integer. Would require " + std::to_string(sum_bits) + " bits.");
             }
         }
         // sentinel
@@ -211,11 +212,15 @@ template <storage::Ownership Ownership> class MultiLevelPartitionImpl final
                             [&](const auto offset, const auto next_offset) {
                                 // create mask that has `bits` ones at its LSBs.
                                 // 000011
-                                BOOST_ASSERT(offset < NUM_PARTITION_BITS);
+                                BOOST_ASSERT(offset <= NUM_PARTITION_BITS);
                                 PartitionID mask = (1ULL << offset) - 1ULL;
                                 // 001111
-                                BOOST_ASSERT(next_offset < NUM_PARTITION_BITS);
-                                PartitionID next_mask = (1ULL << next_offset) - 1ULL;
+                                BOOST_ASSERT(next_offset <= NUM_PARTITION_BITS);
+                                // Check offset for shift overflow. Offsets are strictly increasing,
+                                // so we only need the check on the last mask.
+                                PartitionID next_mask = next_offset == NUM_PARTITION_BITS
+                                                            ? -1ULL
+                                                            : (1ULL << next_offset) - 1ULL;
                                 // 001100
                                 masks[lidx++] = next_mask ^ mask;
                             });
diff --git a/unit_tests/partitioner/multi_level_partition.cpp b/unit_tests/partitioner/multi_level_partition.cpp
index d25664525e9..7fc7acd1452 100644
--- a/unit_tests/partitioner/multi_level_partition.cpp
+++ b/unit_tests/partitioner/multi_level_partition.cpp
@@ -1,6 +1,8 @@
 #include <boost/numeric/conversion/cast.hpp>
 #include <boost/test/unit_test.hpp>
 
+#include "util/exception.hpp"
+#include "util/for_each_indexed.hpp"
 #include <util/integer_range.hpp>
 #include <util/msb.hpp>
 
@@ -230,4 +232,59 @@ BOOST_AUTO_TEST_CASE(large_cell_number)
     }
 }
 
+BOOST_AUTO_TEST_CASE(cell_64_bits)
+{
+    // bits = ceil(log2(2458529 + 1)) + ceil(log2(258451 + 1)) +  ceil(log2(16310 + 1)) +
+    // ceil(log2(534 + 1))
+    //      = 22 + 18 + 14 + 10
+    //      = 64
+    const size_t NUM_PARTITIONS = 2458529;
+    const std::vector<size_t> level_cells = {NUM_PARTITIONS, 258451, 16310, 534};
+    std::vector<std::vector<CellID>> levels(level_cells.size(),
+                                            std::vector<CellID>(level_cells[0]));
+    std::vector<uint32_t> levels_to_num_cells(level_cells.size());
+
+    const auto set_level_cells = [&](size_t level, auto const num_cells) {
+        for (auto val : util::irange<size_t>(0ULL, NUM_PARTITIONS))
+        {
+            levels[level][val] = std::min(val, num_cells - 1);
+        }
+        levels_to_num_cells[level] = num_cells;
+    };
+    util::for_each_indexed(level_cells.cbegin(), level_cells.cend(), set_level_cells);
+
+    MultiLevelPartition mlp{levels, levels_to_num_cells};
+
+    BOOST_REQUIRE_EQUAL(mlp.GetNumberOfCells(1), level_cells[0]);
+    BOOST_REQUIRE_EQUAL(mlp.GetNumberOfCells(2), level_cells[1]);
+    BOOST_REQUIRE_EQUAL(mlp.GetNumberOfCells(3), level_cells[2]);
+    BOOST_REQUIRE_EQUAL(mlp.GetNumberOfCells(4), level_cells[3]);
+}
+
+BOOST_AUTO_TEST_CASE(cell_overflow_bits)
+{
+    // bits = ceil(log2(4194304 + 1)) + ceil(log2(262144 + 1)) +  ceil(log2(16384 + 1)) +
+    // ceil(log2(1024 + 1))
+    //      = 23 + 19 + 15 + 11
+    //      = 68
+    const size_t NUM_PARTITIONS = 4194304;
+    const std::vector<size_t> level_cells = {NUM_PARTITIONS, 262144, 16384, 1024};
+    std::vector<std::vector<CellID>> levels(level_cells.size(),
+                                            std::vector<CellID>(level_cells[0]));
+    std::vector<uint32_t> levels_to_num_cells(level_cells.size());
+
+    const auto set_level_cells = [&](size_t level, auto const num_cells) {
+        for (auto val : util::irange<size_t>(0ULL, NUM_PARTITIONS))
+        {
+            levels[level][val] = std::min(val, num_cells - 1);
+        }
+        levels_to_num_cells[level] = num_cells;
+    };
+    util::for_each_indexed(level_cells.cbegin(), level_cells.cend(), set_level_cells);
+
+    BOOST_REQUIRE_EXCEPTION(MultiLevelPartition(levels, levels_to_num_cells),
+                            util::exception,
+                            [](auto) { return true; });
+}
+
 BOOST_AUTO_TEST_SUITE_END()