MPs cannot stop a no-deal Brexit if they reject May’s agreement or an alternative, according to Commons Leader Andrea Leadsom.
The Brexiteer also warned that those who support a second referendum risk driving voters away from the Conservatives and sparking support for "extremist parties".
The Commons Leader supports a “managed no-deal”, which would see the UK use EU rules during a transitional period while making several mini deals on aviation and trade.
In an interview with the Sunday Telegraph, she said that if parliament does not want a no-deal, MPs should vote for the prime minister's plans.
Three Tory MPs - Nick Boles, Anna Soubry and Sarah Wollaston - have warned they would resign the whip if a no-deal Brexit ever became official Government policy, and Justice Secretary David Gauke has suggested he would quit the Government.
Mrs Leadsom, who publicly supports Theresa May's Brexit deal, told the Sunday Telegraph: "Parliament can always of course express a view, but in order to avoid a no-deal Brexit, Parliament does need to vote for a deal.
"The legal default position is that if there isn't a deal then the UK leaves the EU in March 2019 without a deal."
Theresa May's Withdrawal Agreement is due to be voted on by MPs in January.
But few expect it to be approved by a heavily divided Commons, and Tory MPs including Cabinet ministers appear to have begun to polarise between those who might support a no-deal Brexit and those who would actively battle against it.
However, Lib Dem MP Layla Moran, who supports the Best for Britain group backing a second referendum, insisted that MPs do have the power to stop a no-deal Brexit.
She said: "Is this really where the Brexiters have ended up? The merchants of 'take back control' have pivoted to 'you can't stop us from crashing the car'.
"This is a barking dog that will never bite. The reality is that Parliament can, and will, stop a no-deal Brexit. No politician worth their salt would allow such chaos and unnecessary harm."
