Theresa May was responding to bizarre claims from Jeremy Corbyn it was Labour who had come up with the idea of a transition period.
She said: “The concept of a smooth and orderly exit was first mentioned in my speech at Lancaster House.
“I seem to remember that the day after the referendum result in 2016 the Right Honourable Gentleman wanted to trigger Article 50 immediately! No suggestion of an implementation period then, so there we go.
“It is this Conservative Party in Government that is getting on with delivering the will of the British people.”
Mrs May could be seen laughing when Mr Corbyn had said an implementation period had first been mooted by Labour, while she also attacked his initial call to trigger Article 50 immediately after the referendum.
She added: “It’s the right honourable gentleman who when the shadow home secretary backed a rerun of the referendum, kept her in her job, when the then shadow Northern Ireland secretary backed a rerun of the referendum, he was sacked.”
She said an implementation period was essential to ensure a smooth transition out of the EU for Britain’s businesses.
Mrs May said: “It is not in our national interest to ask businesses to undertake two sets of changes.
“So it follows that during the implementation period they should continue to trade on current terms.
“Whilst I recognise that not everyone will welcome the continuation of current trading terms for another 21 months, such an implementation period has been widely welcomed by British business because it is necessary if we are to minimise uncertainty and deliver a smooth and successful Brexit.”
Mrs May also hit out at the terms of the ‘backstop agreement’ proposed by the EU for Northern Ireland.
She said it risked “destroying the UK internal market” and said the conditions proposed by Brussels were “not acceptable”.
The PM said: “They were not a fair reflection of the joint report.”
But she praised the progress made regarding Brexit and the approval of the Brexit transition period.
During her address in Parliament today, Mrs May also acknowledged the joint EU response to the Russia crisis.
She said Vladimir Putin was overseeing a “new and dangerous phrase” in Europe and more needed to be done to stop him.
Mrs May said: “President Putin’s regime is carrying out acts of aggression against our shared values.”