diff --git a/src/Writers/UblWriter.php b/src/Writers/UblWriter.php
index 9c5d4ba..92216e0 100644
--- a/src/Writers/UblWriter.php
+++ b/src/Writers/UblWriter.php
@@ -14,6 +14,7 @@
 use Einvoicing\Payments\Payment;
 use Einvoicing\Payments\Transfer;
 use UXML\UXML;
+use function in_array;
 
 class UblWriter extends AbstractWriter {
     const NS_INVOICE = "urn:oasis:names:specification:ubl:schema:xsd:Invoice-2";
@@ -168,8 +169,16 @@ public function export(Invoice $invoice): string {
 
         // Invoice lines
         $lines = $invoice->getLines();
-        foreach ($lines as $i=>$line) {
-            $this->addLineNode($xml, $line, $i+1, $invoice); // @phan-suppress-current-line PhanPartialTypeMismatchArgument
+        $lastGenId = 0;
+        $usedIds = [];
+        foreach ($lines as $line) {
+            $lineId = $line->getId();
+            if ($lineId !== null) {
+                $usedIds[] = $lineId;
+            }
+        }
+        foreach ($lines as $line) {
+            $this->addLineNode($xml, $line, $invoice, $lastGenId, $usedIds);
         }
 
         return $xml->asXML();
@@ -745,16 +754,23 @@ private function addDocumentTotalsNode(UXML $parent, InvoiceTotals $totals) {
 
     /**
      * Add invoice line
-     * @param UXML        $parent  Parent XML element
-     * @param InvoiceLine $line    Invoice line
-     * @param int         $index   Invoice line index
-     * @param Invoice     $invoice Invoice instance
+     * @param UXML        $parent     Parent XML element
+     * @param InvoiceLine $line       Invoice line
+     * @param Invoice     $invoice    Invoice instance
+     * @param int         &$lastGenId Last used auto-generated ID
+     * @param string[]    &$usedIds   Used invoice line IDs
      */
-    private function addLineNode(UXML $parent, InvoiceLine $line, int $index, Invoice $invoice) {
+    private function addLineNode(UXML $parent, InvoiceLine $line, Invoice $invoice, int &$lastGenId, array &$usedIds) {
         $xml = $parent->add('cac:InvoiceLine');
 
         // BT-126: Invoice line identifier
-        $xml->add('cbc:ID', $line->getId() ?? (string) $index);
+        $lineId = $line->getId();
+        if ($lineId === null) {
+            do {
+                $lineId = (string) ++$lastGenId;
+            } while (in_array($lineId, $usedIds));
+        }
+        $xml->add('cbc:ID', $lineId);
 
         // BT-127: Invoice line note
         $note = $line->getNote();
diff --git a/tests/Integration/peppol-allowance.xml b/tests/Integration/peppol-allowance.xml
index ccec97a..afd415e 100644
--- a/tests/Integration/peppol-allowance.xml
+++ b/tests/Integration/peppol-allowance.xml
@@ -275,7 +275,7 @@
         </cac:Price>
     </cac:InvoiceLine>
     <cac:InvoiceLine>
-        <cbc:ID>3</cbc:ID>
+        <cbc:ID>a-custom-identifier</cbc:ID>
         <cbc:Note>Testing note on line level</cbc:Note>
         <cbc:InvoicedQuantity unitCode="C62">10</cbc:InvoicedQuantity>
         <cbc:LineExtensionAmount currencyID="EUR">909</cbc:LineExtensionAmount>
diff --git a/tests/Readers/UblReaderTest.php b/tests/Readers/UblReaderTest.php
index e97de37..ae1716c 100644
--- a/tests/Readers/UblReaderTest.php
+++ b/tests/Readers/UblReaderTest.php
@@ -18,6 +18,11 @@ protected function setUp(): void {
     public function testCanReadInvoice(): void {
         $invoice = $this->reader->import(file_get_contents(self::DOCUMENT_PATH));
         $invoice->validate();
+
+        $lines = $invoice->getLines();
+        $this->assertEquals('1', $lines[0]->getId());
+        $this->assertEquals('2', $lines[1]->getId());
+
         $totals = $invoice->getTotals();
         $this->assertEquals(1300, $totals->netAmount);
         $this->assertEquals(1325, $totals->taxExclusiveAmount);
diff --git a/tests/Writers/UblWriterTest.php b/tests/Writers/UblWriterTest.php
index fdd9972..2623e10 100644
--- a/tests/Writers/UblWriterTest.php
+++ b/tests/Writers/UblWriterTest.php
@@ -12,10 +12,12 @@
 use Einvoicing\Presets\Peppol;
 use Einvoicing\Writers\UblWriter;
 use PHPUnit\Framework\TestCase;
+use UXML\UXML;
 use const CURLOPT_HTTPHEADER;
 use const CURLOPT_POSTFIELDS;
 use const CURLOPT_RETURNTRANSFER;
 use const CURLOPT_URL;
+use function array_map;
 use function curl_close;
 use function curl_exec;
 use function curl_init;
@@ -79,7 +81,7 @@ private function getSampleInvoice(): Invoice {
             ->addLine($complexLine)
             ->addLine((new InvoiceLine)->setName('Line #2')->setPrice(40, 2)->setVatRate(21)->setQuantity(4))
             ->addLine((new InvoiceLine)->setName('Line #3')->setPrice(0.56)->setVatRate(10)->setQuantity(2))
-            ->addLine((new InvoiceLine)->setId('5')->setName('Line #4')->setPrice(0.56)->setVatRate(10)->setQuantity(2))
+            ->addLine((new InvoiceLine)->setName('Line #4')->setPrice(0.56)->setVatRate(10)->setQuantity(2))
             ->addAllowance((new AllowanceOrCharge)->setReason('5% discount')->setAmount(5)->markAsPercentage()->setVatRate(21))
             ->addAttachment((new Attachment)->setId(new Identifier('INV-123', 'ABT')))
             ->addAttachment($externalAttachment)
@@ -125,4 +127,27 @@ public function testCanGenerateValidInvoice(): void {
         $contents = $this->writer->export($invoice);
         $this->assertTrue($this->validateInvoice($contents, 'ubl'));
     }
+
+    public function testCanHaveLinesWithForcedDuplicateIdentifiers(): void {
+        $invoice = $this->getSampleInvoice();
+        $invoice->getLines()[1]->setId('DuplicateId');
+        $invoice->getLines()[2]->setId('DuplicateId');
+        $invoice->getLines()[3]->setId('DuplicateId');
+        $xml = UXML::fromString($this->writer->export($invoice));
+        $actualLineIds = array_map(function(UXML $item) {
+            return $item->asText();
+        }, $xml->getAll('cac:InvoiceLine/cbc:ID'));
+        $this->assertEquals(['1', 'DuplicateId', 'DuplicateId', 'DuplicateId'], $actualLineIds);
+    }
+
+    public function testCanAutogenerateInvoiceLineIdentifiers(): void {
+        $invoice = $this->getSampleInvoice();
+        $invoice->getLines()[1]->setId('1');
+        $invoice->getLines()[2]->setId('AnotherCustomId');
+        $xml = UXML::fromString($this->writer->export($invoice));
+        $actualLineIds = array_map(function(UXML $item) {
+            return $item->asText();
+        }, $xml->getAll('cac:InvoiceLine/cbc:ID'));
+        $this->assertEquals(['2', '1', 'AnotherCustomId', '3'], $actualLineIds);
+    }
 }