Pro-Brexit Conservative MP Stephen Phillips has resigned with immediate effect due to "irreconcilable policy differences with the current Government".
Phillips, who has repeatedly called for Theresa May to reveal her Brexit plan to Parliament before beginning the formal process of leaving the European Union, said he was "unable to properly represent" his constituents.
His resignation will trigger a by-election in his Sleaford and North Hykeham seat, where he had a majority of more than 24,000 at last year's general election.
It comes after the High Court ruled that the Prime Minister must get parliamentary approval before triggering Article 50 of the Lisbon Treaty to begin the formal process of leaving the EU, a decision against which the Government is appealing.
In a statement, Phillips said: "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.
"This 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 of my constituents."
A Conservative Party spokesman said: "Stephen Phillips has been a valuable member of Parliament since 2010 and we are sorry that he has chosen to step down. We thank him for his hard work and we wish him every success in the future."
Phillips, who backed Leave in the June 23 referendum, has previously warned against the "tyranny" of denying MPs a vote on the Government's stance in forthcoming Brexit negotiations.
Last month he called for an urgent debate in the House of Commons on the matter, insisting that bypassing Parliament was "simply not an acceptable way for the executive to proceed".
His resignation will heap more pressure on May, who is trying to restore her grip on Brexit amid calls for an emergency general election to settle the question of British withdrawal from the EU.