A Tory peer said the Lords could stall the process of Britain leaving the EU and may even attempt to scrap it.
Baroness Wheatcroft said the unelected Lords are likely to delay any bill to leave the EU, buying time for a movement in favour of a second referendum to build momentum.
Downing Street, however, has dismissed Wheatcroft's comments, repeating its vague mantra, "Brexit means Brexit."
The peer's remarks come as a high-profile Brexit campaigner admitted the EU referendum result was a shock to the Leave campaign, which did not expect to win.
Former Sunday Telegraph editor Patience Wheatcroft said she hoped a pause in introducing article 50 could lead to a second referendum and an eventual vote to stay in the EU.
If it comes to a bill, I think the Lords might actually delay things. I think there's a majority in the Lords for remaining," she told the Times newspaper.
"I would hope, while we delayed things, that there would be sufficient movement in the EU to justify putting it to the electorate, either through a general election or a second referendum."
Lady Wheatcroft said that she did not want the Lords to stand in the way of the UK leaving the EU at the moment, but added: "However, if it comes to a bill, I think the Lords might actually delay things. I think there's a majority in the Lords for remaining."
Asked whether she would support peers delaying Brexit legislation she said: "Yes I would.
"And I would hope, while we delayed things, that there would be sufficient movement in the EU to justify putting it to the electorate, either through a general election or a second referendum."
UK Prime Minister Theresa May has insisted that "Brexit means Brexit" and that her focus is on securing the best possible deal for the UK outside the EU.
Responding to Wheatcroft's comments, a spokeswoman for the Prime Minister said everyone should get behind Brexit.
Lord Bridges of Headley, Minister for Exiting the EU, told peers in July: "The government's position is that there is no legal obligation to consult Parliament on triggering article 50", since "it affects the position in international law and not in domestic law".
Constitutional historian and Conservative Lord Norton of Louth told the BBC that even if the courts find that it is not a prerogative power, "It would be wholly inappropriate for either House to try to prevent the government from implementing the result of the referendum".