Jacob Rees-Mogg accused the Prime Minister of using the implementation period as time for future negotiations, rather than to implement the agreements made in the Brexit negotiations.
The Tory Brexiteer noted: "When the implementation period was announced, it was going to be the implementation of what was agreed upon.
"Now it seems to be a period for further negotiation, without being sure where we're going.
"Does the prime minister know where we are going?"
Theresa May replied: "We have established our plan, we are negotiating on the basis of that plan with the European Union.
"Significant elements of the structure and scope of the future relationship have been agreed upon.
"The legal position, as I am sure my honorable friend knows very well, is that we can not sign a legal text in relation to that future relationship, until we have left the European Union."
"That is why we, and I have also said all the time, that when Parliament examines the retirement agreement, it will also want to have enough details about the future relationship to know what that future relationship will be.
"That's what we're working for."
In a statement from the House of Commons, the prime minister responded to her party's critics by insisting that any power to extend the United Kingdom's transition outside the EU would only be used as a last resort.
The statement follows the anger among the Euro-skeptics Tories in their latest attempt at engagement with Brussels in the search for a Brexit breakthrough.
At the meeting of EU leaders, the Prime Minister held talks on the possible extension of the "implementation period" after Brexit, when the United Kingdom will remain closely linked to the bloc beyond the current final date of December 2020.
In defense of the measure, she told the parliamentarians that the proposal was an "insurance policy".
Ms. May said: "If by the end of 2020 our future relationship is not ready, the proposal is for the UK to make a sovereign choice between UK-wide customs support or a short extension of the implementation period." she said.
"I have not committed to extend the implementation period.
"I do not want to extend the implementation period, and I do not think the extension is necessary."