Parliament must be able to scrutinise the Brexit process and be given the final say over any treaties covering the new relationship between the UK and European Union, peers have said.
A House of Lords committee said the success or failure of the talks between the UK and the EU will have "profound and lasting implications" for the nation and it is "inconceivable" that there should not be parliamentary oversight.
The Lords EU Committee also stressed that any new treaties arising from the talks would need to be ratified by Parliament, and MPs and peers would also have to enact legislation giving effect to Brexit.
The committee also said that the scrutiny would provide an "audit trail" for future generations to see how the most important diplomatic task the Government has undertaken "since the Second World War" is carried out.
The committee's report said: "Parliament's role in the forthcoming negotiations on withdrawal from the EU will be critical to their success: ratification of any treaties arising out of the negotiations will require parliamentary approval, while national legislation giving effect to the withdrawal and new relationship will need to be enacted by both Houses.
"Parliament has a duty to scrutinise and hold the Government to account for decisions that will profoundly affect the United Kingdom.
"It will also be a vital forum for public debate and challenge, on the many issues that will arise in the course of negotiations."
The peers said parliamentary scrutiny should begin even before the Article 50 process has commenced and continue during the complex negotiations during the two-year countdown to Brexit that follows.
The committee said: "Withdrawal from the EU is arguably the most complex, demanding and important administrative and diplomatic task that the Government has undertaken since the Second World War."
Committee chairman Lord Boswell said: "In this report we conclude that it's vital that Parliament is involved in scrutinising every step of the withdrawal process, including any informal discussions that may precede Article 50, formal negotiations under Article 50 itself, and any continuing negotiations establishing a new relationship between the UK and the EU.
"The negotiations are of immense significance to the future of the UK. The rights of EU and UK citizens currently living in each other's countries, our internal security, and the long-term prosperity of the UK are all going to be shaped by these negotiations.
"We can't afford to get this wrong, and we urge the Government to ensure that effective scrutiny is placed at the heart of its plans. We also believe that the House of Lords European Union Committee is in a strong position to carry out this oversight."