diff --git a/changelog.md b/changelog.md
index bbe5261249..0943d78364 100644
--- a/changelog.md
+++ b/changelog.md
@@ -20,6 +20,8 @@
 - The parser now warns about inconsistent spacing around binary operators as
   these can easily be confused with unary operators. This warning will likely
   become an error in the future.
+- The ``'c`` and ``'C'`` prefix for octal literals is now deprecated to
+  bring the language in line with the standard library (e.g. ``parseOct``).
 
 
 #### Breaking changes in the standard library
diff --git a/compiler/lexer.nim b/compiler/lexer.nim
index 375bf227dd..9727d16a7e 100644
--- a/compiler/lexer.nim
+++ b/compiler/lexer.nim
@@ -359,9 +359,8 @@ proc getNumber(L: var TLexer, result: var TToken) =
       inc(pos)
     L.bufpos = pos
 
-  proc lexMessageLitNum(L: var TLexer, msg: string, startpos: int) =
+  proc lexMessageLitNum(L: var TLexer, msg: string, startpos: int, msgKind = errGenerated) =
     # Used to get slightly human friendlier err messages.
-    # Note: the erroneous 'O' char in the character set is intentional
     const literalishChars = {'A'..'F', 'a'..'f', '0'..'9', 'X', 'x', 'o', 'O',
       'c', 'C', 'b', 'B', '_', '.', '\'', 'd', 'i', 'u'}
     var msgPos = L.bufpos
@@ -380,7 +379,7 @@ proc getNumber(L: var TLexer, result: var TToken) =
       add(t.literal, L.buf[L.bufpos])
       matchChars(L, t, {'0'..'9'})
     L.bufpos = msgPos
-    lexMessage(L, errGenerated, msg % t.literal)
+    lexMessage(L, msgKind, msg % t.literal)
 
   var
     startpos, endpos: int
@@ -388,7 +387,8 @@ proc getNumber(L: var TLexer, result: var TToken) =
     isBase10 = true
     numDigits = 0
   const
-    baseCodeChars = {'X', 'x', 'o', 'c', 'C', 'b', 'B'}
+    # 'c', 'C' is deprecated
+    baseCodeChars = {'X', 'x', 'o', 'b', 'B', 'c', 'C'}
     literalishChars = baseCodeChars + {'A'..'F', 'a'..'f', '0'..'9', '_', '\''}
     floatTypes = {tkFloatLit, tkFloat32Lit, tkFloat64Lit, tkFloat128Lit}
   result.tokType = tkIntLit   # int literal until we know better
@@ -398,17 +398,27 @@ proc getNumber(L: var TLexer, result: var TToken) =
   tokenBegin(result, startPos)
 
   # First stage: find out base, make verifications, build token literal string
-  if L.buf[L.bufpos] == '0' and L.buf[L.bufpos + 1] in baseCodeChars + {'O'}:
+  # {'c', 'C'} is added for deprecation reasons to provide a clear error message
+  if L.buf[L.bufpos] == '0' and L.buf[L.bufpos + 1] in baseCodeChars + {'c', 'C', 'O'}:
     isBase10 = false
     eatChar(L, result, '0')
     case L.buf[L.bufpos]
+    of 'c', 'C':
+      lexMessageLitNum(L,
+                       "$1 will soon be invalid for oct literals; Use '0o' " &
+                       "for octals. 'c', 'C' prefix",
+                       startpos,
+                       warnDeprecated)
+      eatChar(L, result, 'c')
+      numDigits = matchUnderscoreChars(L, result, {'0'..'7'})
     of 'O':
-      lexMessageLitNum(L, "$1 is not a valid number; did you mean octal? Then use one of '0o', '0c' or '0C'.", startpos)
+      lexMessageLitNum(L, "$1 is an invalid int literal; For octal literals " &
+                          "use the '0o' prefix.", startpos)
     of 'x', 'X':
       eatChar(L, result, 'x')
       numDigits = matchUnderscoreChars(L, result, {'0'..'9', 'a'..'f', 'A'..'F'})
-    of 'o', 'c', 'C':
-      eatChar(L, result, 'c')
+    of 'o':
+      eatChar(L, result, 'o')
       numDigits = matchUnderscoreChars(L, result, {'0'..'7'})
     of 'b', 'B':
       eatChar(L, result, 'b')
@@ -511,6 +521,7 @@ proc getNumber(L: var TLexer, result: var TToken) =
           if L.buf[pos] != '_':
             xi = `shl`(xi, 1) or (ord(L.buf[pos]) - ord('0'))
           inc(pos)
+      # 'c', 'C' is deprecated
       of 'o', 'c', 'C':
         result.base = base8
         while pos < endpos:
diff --git a/doc/manual.rst b/doc/manual.rst
index 28fbfb1990..5ca6da22e1 100644
--- a/doc/manual.rst
+++ b/doc/manual.rst
@@ -413,7 +413,7 @@ Numerical constants are of a single type and have the form::
   bindigit = '0'..'1'
   HEX_LIT = '0' ('x' | 'X' ) hexdigit ( ['_'] hexdigit )*
   DEC_LIT = digit ( ['_'] digit )*
-  OCT_LIT = '0' ('o' | 'c' | 'C') octdigit ( ['_'] octdigit )*
+  OCT_LIT = '0' 'o' octdigit ( ['_'] octdigit )*
   BIN_LIT = '0' ('b' | 'B' ) bindigit ( ['_'] bindigit )*
 
   INT_LIT = HEX_LIT
@@ -444,7 +444,7 @@ Numerical constants are of a single type and have the form::
 
 As can be seen in the productions, numerical constants can contain underscores
 for readability. Integer and floating point literals may be given in decimal (no
-prefix), binary (prefix ``0b``), octal (prefix ``0o`` or ``0c``) and hexadecimal
+prefix), binary (prefix ``0b``), octal (prefix ``0o``) and hexadecimal
 (prefix ``0x``) notation.
 
 There exists a literal for each numerical type that is
diff --git a/lib/posix/posix.nim b/lib/posix/posix.nim
index 97b4124ec4..c230e65985 100644
--- a/lib/posix/posix.nim
+++ b/lib/posix/posix.nim
@@ -36,26 +36,26 @@
 #      platforms - where do they come from and why are they here?
 when false:
   const
-    C_IRUSR = 0c000400 ## Read by owner.
-    C_IWUSR = 0c000200 ## Write by owner.
-    C_IXUSR = 0c000100 ## Execute by owner.
-    C_IRGRP = 0c000040 ## Read by group.
-    C_IWGRP = 0c000020 ## Write by group.
-    C_IXGRP = 0c000010 ## Execute by group.
-    C_IROTH = 0c000004 ## Read by others.
-    C_IWOTH = 0c000002 ## Write by others.
-    C_IXOTH = 0c000001 ## Execute by others.
-    C_ISUID = 0c004000 ## Set user ID.
-    C_ISGID = 0c002000 ## Set group ID.
-    C_ISVTX = 0c001000 ## On directories, restricted deletion flag.
-    C_ISDIR = 0c040000 ## Directory.
-    C_ISFIFO = 0c010000 ##FIFO.
-    C_ISREG = 0c100000 ## Regular file.
-    C_ISBLK = 0c060000 ## Block special.
-    C_ISCHR = 0c020000 ## Character special.
-    C_ISCTG = 0c110000 ## Reserved.
-    C_ISLNK = 0c120000 ## Symbolic link.</p>
-    C_ISSOCK = 0c140000 ## Socket.
+    C_IRUSR = 0o000400 ## Read by owner.
+    C_IWUSR = 0o000200 ## Write by owner.
+    C_IXUSR = 0o000100 ## Execute by owner.
+    C_IRGRP = 0o000040 ## Read by group.
+    C_IWGRP = 0o000020 ## Write by group.
+    C_IXGRP = 0o000010 ## Execute by group.
+    C_IROTH = 0o000004 ## Read by others.
+    C_IWOTH = 0o000002 ## Write by others.
+    C_IXOTH = 0o000001 ## Execute by others.
+    C_ISUID = 0o004000 ## Set user ID.
+    C_ISGID = 0o002000 ## Set group ID.
+    C_ISVTX = 0o001000 ## On directories, restricted deletion flag.
+    C_ISDIR = 0o040000 ## Directory.
+    C_ISFIFO = 0o010000 ##FIFO.
+    C_ISREG = 0o100000 ## Regular file.
+    C_ISBLK = 0o060000 ## Block special.
+    C_ISCHR = 0o020000 ## Character special.
+    C_ISCTG = 0o110000 ## Reserved.
+    C_ISLNK = 0o120000 ## Symbolic link.</p>
+    C_ISSOCK = 0o140000 ## Socket.
 
 const
   MM_NULLLBL* = nil
diff --git a/tests/lexer/tintegerliterals.nim b/tests/lexer/tintegerliterals.nim
new file mode 100644
index 0000000000..7420db144d
--- /dev/null
+++ b/tests/lexer/tintegerliterals.nim
@@ -0,0 +1,9 @@
+# test the valid literals
+assert 0b10 == 2
+assert 0B10 == 2
+assert 0x10 == 16
+assert 0X10 == 16
+assert 0o10 == 8
+# the following is deprecated:
+assert 0c10 == 8
+assert 0C10 == 8
diff --git a/tests/lexer/tinvalidintegerliteral3.nim b/tests/lexer/tinvalidintegerliteral3.nim
new file mode 100644
index 0000000000..9c2fe69df5
--- /dev/null
+++ b/tests/lexer/tinvalidintegerliteral3.nim
@@ -0,0 +1,7 @@
+discard """
+  file: "tinvalidintegerliteral3.nim"
+  line: 7
+  errormsg: "0O5 is an invalid int literal; For octal literals use the '0o' prefix."
+"""
+
+echo 0O5
diff --git a/tests/misc/tunsignedmisc.nim b/tests/misc/tunsignedmisc.nim
index fc02eee195..b2a3849cf6 100644
--- a/tests/misc/tunsignedmisc.nim
+++ b/tests/misc/tunsignedmisc.nim
@@ -9,7 +9,7 @@ discard """
 # 8 bit
 let ref1 = 128'u8 shr 7
 let hex1 = 0x80'u8 shr 7
-let oct1 = 0c200'u8 shr 7
+let oct1 = 0o200'u8 shr 7
 let dig1 = 0b10000000'u8 shr 7
 
 doAssert(ref1 == 1)
@@ -20,7 +20,7 @@ doAssert(ref1 == dig1)
 # 16 bit
 let ref2 = 32768'u16 shr 15
 let hex2 = 0x8000'u16 shr 15
-let oct2 = 0c100000'u16 shr 15
+let oct2 = 0o100000'u16 shr 15
 let dig2 = 0b1000000000000000'u16 shr 15
 
 doAssert(ref2 == 1)
@@ -31,7 +31,7 @@ doAssert(ref2 == dig2)
 # 32 bit
 let ref3 = 2147483648'u32 shr 31
 let hex3 = 0x80000000'u32 shr 31
-let oct3 = 0c20000000000'u32 shr 31
+let oct3 = 0o20000000000'u32 shr 31
 let dig3 = 0b10000000000000000000000000000000'u32 shr 31
 
 doAssert(ref3 == 1)