The following answers have been received from the offices of grand jurists:
Office of Grand Ayatollah Khamenei (may Allah grant you long life):
If it forms obstruction for water preventing it from reaching skin during Wudhu or Ghusl, it should be removed. In case, the tattoo is under the skin, there would be no problem.
Office of Grand Ayatollah Makarem Shirazi (may Allah grant you long life):
They do not form any obstruction for wudhu.
Office of Grand Ayatollah Saafi Gulpaigani (may Allah grant you long life):
If tattoos prevent water from reaching the skin during wudhu and ghusl, it is not permissible.