amendment made at Committee Stage.
Here is the amended version. The amendment is aimed at securing Parliamentary approval for the outcome of Article 50 TEU negotiations with the EU. The prior approval of both Houses of Parliament shall also be required in relation to any decision by the Prime Minister that the United Kingdom shall leave the European Union without an agreement as to the applicable terms.
The Bill will return to the House of Commons which may or may not accept this amendment. The government does not wish the amendment to be in the Bill because they claim it will harm the UK's negotiating position. For a contrary view see Jolyon Maugham QC's blog Waiting for Godot.
It may be worth noting that Article 50(2) requires the European Parliament to consent to a Withdrawal Agreement. Without consent, the European Council cannot conclude such an agreement.
The question of whether the UK could unilaterally revoke an Article 50 notice has not (yet) been answered. On this see The Three Knights Opinion.