Parliament should be given a vote on triggering formal negotiations to take the UK out of the European Union, a committee of peers has concluded.
It would be "constitutionally inappropriate" and set a "disturbing precedent" for Prime Minister Theresa May to kick off the talks by invoking Article 50 of the EU treaties without first obtaining the approval of both Houses of Parliament, said the House of Lords Constitution Committee.
Although Downing Street has indicated that MPs will have "a say" on Article 50, Mrs May has made clear she believes she can press ahead without a vote by using her prerogative powers.
In a report on Article 50, the Constitution Committee challenged this procedure, arguing that Parliament must "play a central role" in the decision to launch negotiations, either by passing an Act of Parliament or approving resolutions tabled in both Commons and Lords.
And the committee said MPs and peers should also have a key role in scrutinising the Brexit negotiations - due to take two years - and approving the final deal reached between the UK and the remaining 27 EU states.
It was "unclear" whether the UK could unilaterally halt Article 50 talks once they have been triggered, if it changed its mind about wanting to leave the EU, warned the committee.
Parliament should therefore act on the assumption that the Article is "irreversible" and should allow it to be invoked only when it is in the UK's best interests to begin negotiations, said the report.
Committee chairman Lord Lang of Monkton said: "The referendum result was clear and it is right that the Government are preparing to take Britain out of the EU. However, our constitution is built on the principle of parliamentary sovereignty and the decision to act following the referendum should be taken by Parliament.
"Parliament should be asked to approve the decision to trigger Article 50.
"Parliament's assent could be sought by means of legislation or through resolutions tabled in both Houses of Parliament.
"An Act of Parliament would give greater legal certainty and could be used to enshrine the `constitutional requirements' required by Article 50, allowing for the setting of advantageous pre-conditions regarding the exit negotiations to be met before Article 50 could be triggered.
"A resolution could be simpler and quicker to secure but might not provide the same watertight legal authority. We consider that either would be a constitutionally acceptable means of securing parliamentary approval for the triggering of Article 50.
"Parliament and the Government will need to work together to ensure that the UK achieves the best possible outcome when it withdraws from the EU.
"It is therefore important that Parliament plays a key role in scrutinising the Brexit negotiations once Article 50 is triggered.
"We all want the negotiations to produce a deal that works for both the UK and the EU, and Parliament must be involved in holding the Government to account in delivering that."
Downing Street rejected the peers' demand for a parliamentary vote.
Responding to the Lords report, Mrs May's official spokeswoman said: "We take a different view. The Government has set out clearly its position, which is that this is a decision for Government.
"Both Houses of Parliament decided to put the decision about whether or not we remain a member of the EU in the hands of the British people and we now need to get on with delivering that decision."
Meanwhile, the European Parliament's chief Brexit negotiator Guy Verhofstadt called on Mrs May to move swiftly to trigger Article 50, in order to complete the process before the MEP elections due in May 2019.
The PM has said she will not start the negotiations before the end of this year, meaning the two-year talks would stretch on at least into early 2019.
But Mr Verhofstadt suggested that the process should be completed by the end of 2018.
He said in a tweet: "Brexit should be delivered before 2019, when EU politics enters into new cycle and the European Parliament starts new mandate."
The former PM of Belgium added: "If UK wants access to the Single Market, it must also accept the free movement of citizens. Our four freedoms are inseparable."
Mrs May's spokeswoman said Mr Verhofstadt's comment "doesn't strike me as a particularly illuminating remark".
And she added: "On the timings of these negotiations, there is no update to our position that we won't trigger Article 50 until the end of the year and then there will need to be a negotiation process that follows."