Theresa May has warned peers they would give the European Union an incentive to offer Britain a bad Brexit deal if they pass a further amendment to the Article 50 bill.
The House of Lords is expected to vote on Tuesday on an amendment calling for Westminster to get a "meaningful" vote on the eventual Brexit agreement.
The PM has promised Parliament a vote, but only on a "take it or leave it" basis, which would see the UK leave the EU without a deal if MPs reject the agreement.
A number of peers also want the option of telling ministers to go back to the negotiating table to thrash out a better deal.
Some opposition members believe Mrs May's position that "no deal is better than a bad deal" risks a "cliff-edge" move onto World Trade Organization tariffs which would harm the UK economy.
Mrs May's spokesman said: "She believes we should not commit to any process that would incentivise the EU to offer us a bad deal.
"If we are in a position where any deal negotiated by the Prime Minister could be rejected by MPs, that gives strength potentially to other parties in the negotiation."
Conservative MP Bob Neill has signalled he is one of those prepared to vote against the Government unless Mrs May gives Parliament more say on any Brexit deal.
Former Tory chancellor Lord Lamont has warned Brexit is "under attack" on several fronts and urged peers to "see sense" over their threats.
Lords who support Remain must not use their role scrutinising legislation as cover for opposing the will of the people, said Lord Lamont.
He also raised fears about lawyers "concocting some mysterious challenges" to Britain's withdrawal.
Lord Lamont said: "The result was clear, the question was simple and unambiguous and yet Brexit is under attack on several fronts.
"Some say maybe in the future the British people will change their minds. By that they mean they would like to change their minds for them."