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."