As the UK prepares for a “pivotal moment” towards a future outside the European Union North West MEP and Ukip leader Paul Nuttall says the party will continue to be the "guard dogs of Brexit."

The Prime Minister, Theresa May, has signed the letter that starts the formal exit process and the historic document will be hand-delivered by a senior diplomat to EU chiefs in Brussels.

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Mr Nuttall - pictured above - said: “UKIP is delighted that this is eventually happening. 24 years ago this party was formed to make this a reality so it would be churlish to complain.

"We have been ignored, dismissed insulted and abused, but through determination and with good humour we fought on and we have succeeded.

“The nine months delay has, of course cost the taxpayer additional billions, and has allowed those who would block the democratic will of the people to do all they can to spread discord, but we are here now.

“While negotiations continue we will continue to be the 'guard dogs of Brexit', holding the Government's feet to the fire at home and our MEPs will be working to ensure our friends on the continent do not pull any fast ones."

He added: “We will provide the political threat to ensure no backsliding takes place and ensure that Brexit does indeed mean Exit.”

“With our six key tests we have set out a reasonable and clear position by which the Government's negotiations can be held to account.

“But today for now, we wish the Government and the Prime Minister well.

“If they fail in this historic task, 17 million people are ready to act.”

Shortly before a Cabinet meeting, Chancellor Philip Hammond said the letter sets the “right tone” and sends the “right signals” to European leaders about how the UK wants to conduct the negotiations that will decide the country’s future.

At some time after 12.30pm the premier will inform MPs that Brexit is being triggered and, in Brussels, British ambassador to the EU Sir Tim Barrow will deliver the document to European Council president Donald Tusk.

Once it has been accepted, Article 50 has been officially launched, starting a two-year countdown to the UK leaving the EU.

Mr Hammond said it was an “exciting time”, telling the BBC Radio 4 Today programme: “This is a pivotal moment for Britain.”

He said “Morning” to reporters as he walked into the Europa Building in the heart of the European quarter, where he is expected to hand the document to Mr Tusk this afternoon.

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Mr Hammond said the letter builds on the position the Prime Minister has already set out, indicating that the UK would leave the single market and would not remain a “full member” of the customs union.

“Of course it will go further in expressing how we want to take the negotiation forward and how we see this negotiation developing,” he said.

The Chancellor said the Government’s position on leaving the single market and customs union would have “consequences” but demonstrated that “we understand that we can’t cherry-pick, we can’t have our cake and eat it”.

The Chancellor said that although preparations were being made for the possibility of leaving the EU without a deal, he was “absolutely confident” that an agreement on the ongoing relationship would be sealed.

The Prime Minister will promise to represent “every person in the UK”, including EU nationals, when she takes to the negotiating table.

EU referendum: how the nations voted graphic She will tell the Commons that “as we face the opportunities ahead of us on this momentous journey, our shared values, interests and ambitions can, and must, bring us together”.

Key EU figures agreed to enter into Brexit talks in a “positive spirit” during a series of telephone calls with Mrs May on Tuesday evening.

German Chancellor Angela Merkel, Mr Tusk and European Commission president Jean-Claude Juncker said the UK would remain a “close and committed ally”.

Within 48 hours, the European Commission is expected to issue “draft negotiation guidelines”, which will be sent to the 27 remaining states for consultation.

Their leaders will meet on April 29 at an extraordinary European Council summit to agree a mandate for chief negotiator Michel Barnier and clear the way for talks to begin in earnest in May.