diff --git a/src/Spout/Writer/Internal/XLSX/Worksheet.php b/src/Spout/Writer/Internal/XLSX/Worksheet.php
index f778b8c..edde970 100644
--- a/src/Spout/Writer/Internal/XLSX/Worksheet.php
+++ b/src/Spout/Writer/Internal/XLSX/Worksheet.php
@@ -149,7 +149,7 @@ EOD;
             if (empty($cellValue)) {
                 $data .= '/>' . PHP_EOL;
             } else {
-                if (trim($cellValue, '-0123456789.') == '' /*similar to is_numeric without having PHPs regular quirkiness*/) {
+                if (trim(ltrim($cellValue, '-'), '0123456789.') == '' /*similar to is_numeric without having PHPs regular quirkiness*/) {
                     $data .= '><v>' . $cellValue . '</v></c>' . PHP_EOL;
                 } else {
                     if ($this->shouldUseInlineStrings) {
diff --git a/src/Spout/Writer/XLS.php b/src/Spout/Writer/XLS.php
index 2278783..ac55d5a 100644
--- a/src/Spout/Writer/XLS.php
+++ b/src/Spout/Writer/XLS.php
@@ -164,7 +164,7 @@ class XLS extends AbstractWriter
 
         $this->home();
         foreach ($dataRow as $cell) {
-            if ($cell != '' && trim($cell, '-0123456789.') == '' /*similar to is_numeric without having PHPs regular quirkiness*/) {
+            if ($cell != '' && trim(ltrim($cell, '-'), '0123456789.') == '' /*similar to is_numeric without having PHPs regular quirkiness*/) {
                 $wasWriteSuccessful = $wasWriteSuccessful && $this->number($cell);
             } else
             {