Theresa May has said she will reconsider paying the £39bn Brexit divorce bill if the UK is unable to reach a deal with the EU.
Speaking in Parliament, she said the UK was a law-abiding nation which would honour its international commitments.
But asked by Tory MP Chris Philp what would happen in the event of a no deal, she said the "position changes".
"The specific offer was made in the spirit of our desire to reach an agreement with the EU," she added.
The proposed financial settlement is part of the withdrawal negotiations that both sides hope to complete by November at the latest ahead of the UK's exit next March.
The UK and EU agreed in principle in December that the UK would pay about £39bn to cover outstanding financial obligations and future liabilities arising from its EU membership.
Mrs May was asked by Chris Philp whether the payment was dependent on reaching an agreement on the UK's future relations with the EU that was "satisfactory" to MPs.
He also urged the prime minister not to be "locked into" any financial agreement unless there were guarantees the UK would be free to work on new trade agreements during the proposed transition period ending in December 2020 and to sign them the moment it leaves.
In response, Mrs May said the negotiations were being conducted on the basis that "nothing is agreed until everything is agreed".
"We are very clear we need to have a link between the future relationship and the withdrawal agreement."
Tory MPs have expressed concerns that the UK could find itself paying the money with no guarantees over its future trade relationship, Boris Johnson suggesting the UK has agreed to "hand over £40bn of taxpayers' money for two-thirds of diddly squat".