In a statement on his website, Mr Lamont says he is concerned the deal could undermine the integrity of the UK and result in an “uncomfortable half-way house” of having to follow EU rules without any influence or say over them.
But he added that he opposes both a second referendum and a No-Deal Brexit, and said he would support the withdrawal agreement if the Northern Ireland backstop was removed.
“I believe the withdrawal agreement could harm the integrity of the United Kingdom and help the nationalists in Scotland with their plan to break up Britain,” Mr Lamont said.
The conservative MP added that voting against his party was “not a decision I take lightly”.
“It should not be seen as a vote against the Prime Minister, who I think has done a near impossible task and done it well,” he insisted. “She has been let down by the EU which has not been forthcoming or reasonable despite knowing this deal is unacceptable to the UK Parliament.
“However, I need to put my constituents and my country first. I do not believe that this deal delivers for either.
“Everyone was promised ahead of the EU referendum that their choice would be implemented and that is what should happen.
“While I believe that some of the warnings about a No-Deal scenario have been over exaggerated, No-Deal would clearly risk disruption to our economy and it should be avoided.
“My preference would be for the UK to leave the EU with a deal which delivers the referendum result to leave but which provides stability for people and for businesses. I believe that the majority of MPs want this too.”
Today’s debate will end around 6.20pm tonight with a speech from the prime minister.
MPs will then get a chance to reshape, or reject the deal by voting on a series of amendments to it before the vote on the Brexit agreement happens,