Conservative MP Stephen Phillips has resigned, citing "very significant policy differences" with the current government.
The Conservative MP for Sleaford and North Hykeham, was pro-Leave in the referendum debate, but has since complained the government is avoiding parliamentary scrutiny of Brexit.
Phillips said in a statement: "It has been a great honour to serve the people of Sleaford and North Hykeham for the last six years, but it has become clear to me over the last few months that my growing and very significant policy differences with the current government mean that I am unable properly to represent the people who elected me.
"The decision has been a difficult one and I hope that everyone will respect the fact that I have tried to act in the best interests of all my constituents."
He is quitting with immediate effect.
Phillips was first elected to parliament for the seat in 2010, and retained it in 2015.
Theresa May said earlier this month that parliament would be able to scrutinise her plan for Brexit before Article 50 is invoked, in an effort to quell Tory rebellion. But she didn't want to show all her cards and the Conservatives had said there would be "no running commentary" on their plans.
Only, a spanner was thrown into the works yesterday, when a High Court judge ruled the Brexit process could not be started until parliament had given its backing. As it stands, Theresa May can't trigger Article 50 without approval from MPs. The government said yesterday it would appeal the decision.
The case was brought in the aftermath of the EU referendum to determine whether an Act of Parliament needs to be put in place before government triggers Article 50, or whether government has the prerogative to start the UK's EU withdrawal process by itself.
The case has been brought to court by a number of parties, but the lead claimant is fund boss Gina Miller.