Suzanne Evans has vowed UKIP will be the "guard dog" of the Brexit negotiations ahead of the formal triggering of Article 50.
Shortly after PMQs, the Prime Minister will tell MPs the EU has been formally notified of the United Kingdom's departure from the bloc.
It will mark the start of a two-year negotiation process to redefine the UK's relationship with the European Union.
Evans, the deputy Chair of UKIP, made the vow in an interview with Paul Ross.
She said: "There seem to be these people who just can’t let go, can’t embrace the new reality. We’ll have to see what the negotiations bring, and UKIP has been very clear.
"We want the Brexit vote leave campaigned for, we want control of our legal system, we want control of our borders back, we want to ensure we’re not paying out money to the EU so we can spend it on priorities here at home.
“We will be the guard dogs of Brexit on that. I am optimistic, I think it’ll be fairly rosy.
"At the end of the day, we’ll be free from the clutches of a very domineering, very controlling group of people whom ultimately we have no independence from."