British Prime Minister Boris Johnson will tomorrow unveil his final Brexit offer to the European Union and make clear that if Brussels does not engage with the proposal, Britain will not negotiate further and leave the bloc on 31 October.

In his closing speech to the Conservative party's annual conference, Mr Johnson will stick to his hard line on Brexit, offering the party faithful the first details of what he will describe as his "fair and reasonable compromise".

With less than a month until Britain is due to leave the EU, the future of Brexit, the country's biggest trade and foreign policy shift in more than 40 years, is uncertain.

Mr Johnson, who says Britain will leave the bloc on 31 October no matter what, will tell the conference he will send his proposal to Brussels, an attempt to secure a deal to smooth the country's departure and avoid a potentially damaging no-deal Brexit.

"My friends, I am afraid that after three-and-a-half years people are beginning to feel that they are being taken for fools. They are beginning to suspect that there are forces in this country that simply don't want Brexit delivered at all," he will say, according to extracts released by his office.

"Let's get Brexit done on October 31 so in 2020 our country can move on."

More than three years after Britain voted to leave the EU in a 2016 referendum, Brexit talks are at an impasse.

Mr Johnson has been firm that the end of October deadline will be met, but parliament has put roadblocks in his way - passing a law that requires the prime minister to request a Brexit delay if he fails to secure an acceptable deal at an EU summit on 17 October.

The EU has repeatedly asked Britain to come up with "legal and operational" proposals for the changes Mr Johnson wants to a deal his predecessor negotiated with the bloc last year.

Following its rejection three times by parliament, Mr Johnson has demanded changes to the Withdrawal Agreement.

But after weeks of talks since Mr Johnson took power that have made little headway to break the Brexit stand-off, the prime minister will make his last gambit - a new proposal which British officials describe as a final offer.

The Telegraph newspaper cited a briefing to European capitals that Britain was suggesting a plan that would leave Northern Ireland in a special relationship with the EU until 2025, after which Belfast would decide whether to remain aligned to the bloc or return to following British rules.

The proposal would be aimed at replacing the backstop that has become the biggest hurdle to securing an agreement with Brussels.

Minister for European Affairs Helen McEntee has said that such a proposal would not be accepted. 

Speaking on RTÉ's Prime Time, the minister said: "What we're talking about, again, is picking and choosing certain parts of the single market that would be aligned in Northern Ireland.

"It is talking about a time limit which is not acceptable - but it is still talking about the need and requirement for customs checks."

A senior British government official said: "The government is either going to be negotiating a new deal or working on no deal — nobody will work on delay.

"We will keep fighting to respect the biggest democratic vote in British history. The EU is obliged by EU law only to negotiate with member state governments, they cannot negotiate with parliament, and this government will not negotiate delay."

Mr Johnson has made the gamble that by pressing a hardline position on Brexit he will steal votes from parties such as the Brexit Party led by veteran euro sceptic Nigel Farage at an election, widely expected to come before the end of the year.

He also has the main opposition Labour Party in his sights, and will use his speech to attack its leader, Jeremy Corbyn, who has led efforts to try to stop the prime minister taking Britain out of the EU without a deal.

"Can you imagine another three years of this? That is the Corbyn agenda – stay in the EU beyond October 31, paying £1 billion a month for the privilege, followed by years of uncertainty for business and everyone else," he will say.

"That is why we are coming out of the EU on October 31. Let's get Brexit done - we can, we must and we will."

Johnson says UK prepared to be 'flexible' on Brexit

Earlier, Mr Johnson said that Britain was prepared to be "very flexible" to solve the problem of post-Brexit customs checks on the island of Ireland.

His statement came after Taoiseach Leo Varadkar insisted that the British government should not "impose" customs checks "against the will of the people".

He said: "If you can have Northern Ireland as part of a UK customs union, which is plainly essential, then somehow or other you have to solve the problem of customs checks."

Mr Johnson said the UK was "very, very prepared" to be flexible.

"We don't want to have any kind of checks at all at the border," he said

He said one solution might be to have a relationship between exporters and importers, with checks at either end.

"That I don't think will be onerous, it certainly wouldn't involve infrastructure, but there might be ways to do it."

Mr Johnson also described the chances of reaching an exit deal with the European Union as "good".

Read more: Latest Brexit headlines

Politicians from across the spectrum in Ireland have poured cold water on proposals sent to the EU by the UK would lead to the posts being built between five and ten miles back from the current border.

Mr Johnson said those were preliminary ideas that had been floated and the formal proposals would be set out very shortly - widely expected to be later this week after the Tory conference finishes tomorrow.

In an indication of why the UK has delayed tabling its formal proposals, Mr Johnson said there was a risk that critics could "needlessly distort" what was being put forward.

"We do think think there's a good solution. I very much hope that our European and EU friends in Brussels, in Dublin, in Germany as well, will want to take it forward," he said.

The idea for the customs posts was contained in the so-called "non-papers" submitted by UK officials during recent technical discussions.