diff --git a/src/IntPatch/IntPatch_Intersection.cxx b/src/IntPatch/IntPatch_Intersection.cxx
index 5e02c67621..75b6ba4e52 100644
--- a/src/IntPatch/IntPatch_Intersection.cxx
+++ b/src/IntPatch/IntPatch_Intersection.cxx
@@ -910,9 +910,9 @@ void IntPatch_Intersection::Perform(const Handle(Adaptor3d_HSurface)&  theS1,
   //                              3. ts1 == ts2 == 0 <Param-Param>
 
   // Geom - Geom
+  const Standard_Boolean RestrictLine = Standard_True;
   if(ts1 == ts2 && ts1 == 1)
   {
-    const Standard_Boolean RestrictLine = Standard_True;
     IntSurf_ListOfPntOn2S ListOfPnts;
     ListOfPnts.Clear();
     if(isGeomInt)
@@ -946,7 +946,6 @@ void IntPatch_Intersection::Perform(const Handle(Adaptor3d_HSurface)&  theS1,
   // Param - Param 
   if(ts1 == ts2 && ts1 == 0)
   {
-    const Standard_Boolean RestrictLine = Standard_True;
     IntSurf_ListOfPntOn2S ListOfPnts;
     ListOfPnts.Clear();
 
@@ -964,7 +963,8 @@ void IntPatch_Intersection::Perform(const Handle(Adaptor3d_HSurface)&  theS1,
     if(aWL.IsNull())
       continue;
 
-    Handle(IntPatch_WLine) aRW = IntPatch_WLineTool::ComputePurgedWLine(aWL, theS1, theS2, theD1, theD2);
+    Handle(IntPatch_WLine) aRW =
+      IntPatch_WLineTool::ComputePurgedWLine(aWL, theS1, theS2, theD1, theD2, RestrictLine);
 
     if(aRW.IsNull())
       continue;
@@ -1207,7 +1207,8 @@ void IntPatch_Intersection::Perform(const Handle(Adaptor3d_HSurface)&  theS1,
     if(!aWL->IsPurgingAllowed())
       continue;
 
-    Handle(IntPatch_WLine) aRW = IntPatch_WLineTool::ComputePurgedWLine(aWL, theS1, theS2, theD1, theD2);
+    Handle(IntPatch_WLine) aRW = 
+      IntPatch_WLineTool::ComputePurgedWLine(aWL, theS1, theS2, theD1, theD2, RestrictLine);
 
     if(aRW.IsNull())
       continue;
@@ -1720,7 +1721,8 @@ void IntPatch_Intersection::Perform(const Handle(Adaptor3d_HSurface)&  S1,
     if(aWL.IsNull())
       continue;
 
-    Handle(IntPatch_WLine) aRW = IntPatch_WLineTool::ComputePurgedWLine(aWL, S1, S2, D1, D2);
+    Handle(IntPatch_WLine) aRW =
+      IntPatch_WLineTool::ComputePurgedWLine(aWL, S1, S2, D1, D2, Standard_True);
 
     if(aRW.IsNull())
       continue;
diff --git a/src/IntPatch/IntPatch_WLineTool.cxx b/src/IntPatch/IntPatch_WLineTool.cxx
index 1b823b6ab9..ca38c1c2b1 100644
--- a/src/IntPatch/IntPatch_WLineTool.cxx
+++ b/src/IntPatch/IntPatch_WLineTool.cxx
@@ -1197,7 +1197,8 @@ Handle(IntPatch_WLine) IntPatch_WLineTool::
                      const Handle(Adaptor3d_HSurface)   &theS1,
                      const Handle(Adaptor3d_HSurface)   &theS2,
                      const Handle(Adaptor3d_TopolTool)  &theDom1,
-                     const Handle(Adaptor3d_TopolTool)  &theDom2)
+                     const Handle(Adaptor3d_TopolTool)  &theDom2,
+                     const Standard_Boolean              theRestrictLine)
 {
   Standard_Integer i, k, v, nb, nbvtx;
   Handle(IntPatch_WLine) aResult;
@@ -1300,13 +1301,15 @@ Handle(IntPatch_WLine) IntPatch_WLineTool::
     return aLocalWLine;
   }
 
-  // II: Delete out of borders points.
-  Handle(IntPatch_WLine) aLocalWLineOuter = 
-    DeleteOuterPoints(aLocalWLine, theS1, theS2, theDom1, theDom2);
+  if (theRestrictLine)
+  {
+    // II: Delete out of borders points.
+    aLocalWLine = DeleteOuterPoints(aLocalWLine, theS1, theS2, theDom1, theDom2);
+  }
 
   // III: Delete points by tube criteria.
   Handle(IntPatch_WLine) aLocalWLineTube = 
-    DeleteByTube(aLocalWLineOuter, theS1, theS2);
+    DeleteByTube(aLocalWLine, theS1, theS2);
 
   if(aLocalWLineTube->NbPnts() > 1)
   {
diff --git a/src/IntPatch/IntPatch_WLineTool.hxx b/src/IntPatch/IntPatch_WLineTool.hxx
index 865913c761..90b0d5d71f 100644
--- a/src/IntPatch/IntPatch_WLineTool.hxx
+++ b/src/IntPatch/IntPatch_WLineTool.hxx
@@ -39,6 +39,7 @@ public:
   //!
   //! II
   //! Removes point out of borders in case of non periodic surfaces.
+  //! This step is done only if theRestrictLine is true.
   //!
   //! III
   //! Removes exceed points using tube criteria:
@@ -52,7 +53,8 @@ public:
                                               const Handle(Adaptor3d_HSurface) &theS1,
                                               const Handle(Adaptor3d_HSurface) &theS2,
                                               const Handle(Adaptor3d_TopolTool)  &theDom1,
-                                              const Handle(Adaptor3d_TopolTool)  &theDom2);
+                                              const Handle(Adaptor3d_TopolTool)  &theDom2,
+                                              const Standard_Boolean      theRestrictLine);
 
 //! Joins all WLines from theSlin to one if it is possible and records 
 //! the result into theSlin again. Lines will be kept to be splitted if:
diff --git a/tests/bugs/modalg_6/bug27780 b/tests/bugs/modalg_6/bug27780
new file mode 100644
index 0000000000..4df90be0a9
--- /dev/null
+++ b/tests/bugs/modalg_6/bug27780
@@ -0,0 +1,33 @@
+puts "============"
+puts "OCC27780"
+puts "============"
+puts ""
+######################################################
+# Face-face intersection produces 2D curve that has reversed derivative at its end
+######################################################
+
+restore [locate_data_file buc60532a.brep] p 
+restore [locate_data_file buc60532b.brep] t1
+explode t1 f
+
+bopcurves p t1_2 -2d -p 6.2406621764215551 0.23999999463558200 0.00034444887595448459 -5.0019657458625186
+
+pcurve p
+trim c c2d1_1 0 1
+2dcvalue c 0 u0 v0
+2dcvalue c 1 u1 v1 du dv
+dset u01 u1-u0
+dset v01 v1-v0
+dset dot u01*du+v01*dv
+
+if {[dval dot] < 0} {
+  puts "Error: p-curve has reversed direction at its end"
+} else {
+  puts "OK: p-curve has correct direction at its end"
+}
+
+view 1 -2D- 728 450 400 400
+don p_* c
+2dfit
+
+checkview -screenshot -2d -path ${imagedir}/${test_image}.png