Paul Nuttall said that his party's only MP Douglas Carswell, who quit Ukip yesterday, jumped ship too soon.
The Ukip leader said that the Clacton MP was naive for trusting Theresa May and her colleagues to negotiate the Brexit deal in Britain's best interests.
Mr Carswell announced his departure from Ukip yesterday to become an independent MP for Clacton.
The dramatic move, which comes after months of in-fighting, leaves the Brexit-centric party without a single representative in the House of Commons.
Party leader Paul Nuttall said Mr Carswell was 'committed to Brexit, but never comfortable in Ukip'
Mr Nuttall told Andrew Neil, who was sat in the studio alongside the Clacton MP: "I am over the moon that we have achieved the Brexit.
"But, unlike Douglas, I don't have that much confidence in Theresa May.
"History has proved that she is good at talking the talk but when it comes to walking the walk, she tends to fail."
He condemned Mr Carswell's departure as premature, as he argued that Ukip must stick around to finish the Brexit process and ensure that Theresa May sticks to her word.
Ukip's former MP defended his move, claiming that Theresa May had committed herself to Brexit and so "the job was done".
Earlier this month, Mr Carswell vowed to "absolutely" fight the next general election as a UKIP candidate.
He had previously defected from the Conservatives to the party in 2014.
Get Quotes on Home Insurance
Mr Nuttall added that Ukip will soon unveil a raft of new domestic policies
When it comes to walking the walk, she tends to fail
Paul Nuttall on Theresa May's Brexit policy
Mr Nuttall added that Ukip will soon unveil a raft of new domestic policies in an attempt to recreate a 'post-Brexit Ukip'.
He insisted that the party will focus on taking seats at Westminister.
The party leader also admitted that Mr Carswell was "never a comfortable Ukipper" and his departure was "not a surprise".
There are now calls within Ukip for their former colleague to call a by-election.
So far Mr Carswell has rejected these demands, claiming his constituents are 'election-fatigued'.
He also rejected claims that he will return to the Conservative Party, claiming that he will support Brexit and Mrs May as a "critical friend".
There are now calls within Ukip for the newly-independent MP to call a by-election
Mr Carswell announced his departure from Ukip yesterday, to become an independent MP for Clacton
But, Nigel Farage, the former leader of Ukip, has pledged to write to every single constituent in Clacton to ratchet up the pressure on his party's former MP.
Mr Farage and Mr Carswell have endured an acrimonious relationship in the past, with the former leader of the party recently accusing him of "actively working against UKIP".
UKIP MEP Bill Etheridge said he was "delighted" as the "focus of serious infighting in the party has gone".
He added: "But I think he needs to step down and call a by-election.
"He was elected twice on a UKIP ticket, with UKIP resources and the hard work and shoe leather of UKIP activists."