ADDED: "The referendum result is not legally binding - Parliament still has to pass the laws that will get Britain out of the 28 nation bloc, starting with the repeal of the 1972 European Communities Act."
The withdrawal agreement also has to be ratified by Parliament - the House of Lords and/or the Commons could vote against ratification, according to a House of Commons library report.
In practice, Conservative MPs who voted to remain in the EU would be whipped to vote with the government. Any who defied the whip would have to face the wrath of voters at the next general election.
One scenario that could see the referendum result overturned, is if MPs forced a general election and a party campaigned on a promise to keep Britain in the EU, got elected and then claimed that the election mandate topped the referendum one.