Jacob Rees-Mogg has submitted a letter of no confidence in Prime Minister Theresa May.
A spokesman for Mogg’s pro-Brexit European Research Group confirmed he has submitted the letter, after challenging the Tory leader in parliament over her Brexit deal.
"What has been achieved today is not Brexit," Mogg told reporters this afternoon. "It is impossible to agree to a situation where we have a perpetual customs union."
"This opportunity is being thrown away. Our tariffs will be set by the European Union, it will be illegal to offer lower tariffs. This is not Brexit this is a failure of government policy and it needs to be rejected."
His letter, shared on Twitter by The Times journalist Sam Coates, criticises May's deal for keeping the UK inside a temporary customs union with the EU without the ability for the UK to decide to leave this arrangement.
"It is … harder to leave this backstop than it is to leave the EU," Mogg said.
He also criticised the Prime Minister for agreeing to pay a £39bn so-called divorce bill with the EU, saying there is no legal obligation to pay the bloc.
Mogg also claimed that the UK is not "taking back control of our laws".
"It is of considerable importance that politicians stick to their commitments or do not make such commitments in the first place," Mogg wrote.
The Prime Minister would face a vote on her leadership if 48 no confidence letters are received.
A total 158 Tory MPs – of 315 in total – would have to vote against May for her to stand down as leader.