THERESA May has denied “climbing down” on Brexit but has agreed to a so-called Irish back-stop plan, which some believe will keep Britain in the European customs union beyond the end of the transition period in December 2020.

While the Prime Minister and her senior colleagues are still mulling over two proposed options – a customs partnership and maximum facilitation – for customs arrangements post Brexit and remain confident one of them can be agreed upon, a back-stop plan has been endorsed should neither of them be taken forward by the end of the transition period.

The proposal would mean the whole of the UK would be covered by the EU’s common external tariff, removing any need for a customs border on the island of Ireland or between it and mainland UK.

Brexiteers fear this back-stop plan – agreed this week by Cabinet despite opposition from Boris Johnson, the Foreign Secretary, and Michael Gove, the Environment Secretary - amounts to staying in the customs union longer.

One, Jacob Rees-Mogg, complained: "We have gone from a clear end-point to an extension to a proposed further extension with no end-point.

"The horizon seems to be unreachable. The bottom of the rainbow seems to be unattainable. People voted to leave; they did not vote for a perpetual purgatory," declared the Somerset MP.

But Damian Green, the former Cabinet Office Minister, who was Mrs May's effective deputy until his resignation last year, said he was ready to accept a "small delay" to Brexit to ensure that customs arrangements worked effectively.

In a tweet, the Kent MP said: "Surely the point about a new customs arrangement is that it needs to work smoothly from day one or we will have chaos on the roads, especially in Kent. If that means a small delay, so be it."

No 10 sources insisted the proposed back-stop plan, which will be presented to the EU27 at next month’s European Council, would not be tantamount to Britain staying in the customs union longer, claiming that in such circumstances the UK would still be able to sign and implement trade deals; moreover, any such measure would only last for a matter of months.

Leo Varadkar, the Irish Taoiseach, warned: "We need that [back-stop plan] to be part of the withdrawal agreement and if it’s not, then there will be no withdrawal agreement and no transition period."

Arriving at the EU Western Balkans summit in the Bulgarian capital Sofia, the PM, asked about the back-stop plan, said: “No, we are not [climbing down]. The United Kingdom will be leaving the customs union; we are leaving the European Union.”

She went on: "Of course, we will be negotiating future customs arrangements with the European Union and I have set three objectives…We need to be able to have our own independent trade policy, we want as friction-less a border between the UK and the EU so that trade can continue and we want to ensure there is no hard border between Northern Ireland and Ireland."

Following a trilateral meeting with Donald Tusk, the European Council President, and Jean-Claude Juncker, the European Commission President, and talks with Mr Varadkar, Mrs May told a press conference in the Macedonian capital of Skopje: “The Commission published a fall-back option, which was not acceptable to us and we will be bringing forward our own proposal for that fall-back option in due course."

She acknowledged the Irish Government’s concerns would have to be dealt with so "we do not have a hard border between Northern Ireland and Ireland".

Mrs May added: "That's what we will be doing and ensuring that at the end of the implementation period - as of the end of December 2020 – we will be operating an independent trade policy."

Meanwhile in the Commons, Tory Brexiteer Peter Bone proposed making the date of the EU referendum, June 23, a “Brexit bank holiday,” saying, to laughter from MPs, that it would become known as Britain’s very own “independence day”.