TORY Brexiteers have responded angrily to Theresa May’s suggestion that she might be ready to accept an extension to the post-Brexit transition period as they denounced Brussels’ “blackmail”.

But it was suggested that any agreement by the Prime Minister to extend the 21-month transition period, due to end in December 2020, would be contingent on the EU27 signing up to the UK Government’s version of the backstop ie it has to be UK-wide.

Earlier today, Mrs May confirmed that she and her colleagues were ready to consider a transition period extension but only for “a matter of months”.

However, the PM again stressed that she did not expect any transition period extension would be needed because she still hoped to conclude a deal on the UK's future trade and security relationship with the EU by its scheduled end-date of December 2020.

She told reporters: "A further idea that has emerged - and it is an idea at this stage - is to create an option to extend the implementation period for a matter of months and it would only be for a matter of months.

"But the point is that this is not expected to be used because we are working to ensure that we have that future relationship in place by the end of December 2020.

"I'm clear that it is possible to do that and that is what we are working for. In those circumstances, there would be no need for any proposal of this sort and I'm clear that I expect the implementation period to end at the end of December 2020."

Downing St was later pressed and adopted a similar line, saying only: “There are no detailed proposals. It’s an idea.”

But the implications of an extension riled the Brexiteers.

Sir Bill Cash, the veteran Eurosceptic, tweeted: “Unthinkable that we should be blackmailed by the EU into billions and billions, plus loss of rebate in extended transition after 31st December 2020. Completely unnecessary.” He added: “Time to walk away.”

Jacob Rees-Mogg, who chairs the European Research Group of Eurosceptic Tories, described the proposed extension to the transition as "a poorly thought-through idea".

He noted how going beyond December 2020 would move into the next EU budget phase and the prospect of “very substantial costs because the next framework will be determined without our veto and therefore it is unlikely we would retain our rebate; it could be very expensive to do”.

The Somerset MP pointed out that any transition period extension did not solve the problem of the backstop because it would still remain at the end of this additional period.

"It seems to me to be a rather poor attempt at kicking the can down the road," declared Mr Rees-Mogg.

He added: "The Government doesn't have endless money. It has to make choices. The question the Government has to answer is if this costs billions of pounds, is it better to give the money to the EU or better to sort out the issues with Universal Credit? I would prefer the money to be spent ensuring Universal Credit works."

Underlining how Mrs May is under fire from both sides of her party, Nick Boles, the former minister, who is pushing for a soft Brexit move to temporary membership of the European Free Trade Association, warned: "I'm afraid she is losing the confidence now of colleagues of all shades of opinion.”

He explained: "They are close to despair at the state of this negotiation because there is a fear that both the Government and the European Union are trying to run out the clock, that they are trying to leave this so late that they can credibly say there is no alternative but a no-deal Brexit and most people agree that would be chaos.

"That is not an acceptable way for a leader of a government to behave," he added.

In a separate development, Mr Rees-Mogg together with Boris Johnson and David Davis joined three other former ministers in signing an open letter warning the PM not to "bind the UK into the purgatory of perpetual membership of the EU's customs union".

The group called on her to "reset" the negotiations and ditch her Chequers Brexit blueprint in favour of a Canada-style free trade agreement.

Meanwhile, Tom Brake for the Liberal Democrats said: "Extending the transition? More like kicking the can further down the road and delaying, by a bit, driving off the cliff. No deal the PM can secure will be as good as staying in the EU."