The new measure would keep the 24-weeka abortion limit for doctors but remove criminal penalties for women who carry out abortions themselves at any stage, including late-term procedures.
British pro-life leaders say the legislation, which is now being considered by the House of Lords, is dangerously vague and opens the door to coercion and abuse.
The proposed legislation 'is an instance of the abrogation of responsibility since it will ultimately be the decision of a particular judge to set the precedent for the correct interpretation of the law,' Right to Life UK's Catherine Robinson said.
Baroness Jane Campbell of disability rights group Not Dead Yet said that legislators tried to push the amendment 'by the back door' in a 'blatant manipulation of the parliamentary process.'