The 76-year-old singer will be the first to perform in Azerbaijan’s Crystal Hall, with his ballad Love Will Set You Free.
Sweden’s Euphoria – a club hit by singer Loreen that has already topped the charts in four countries – is among the frontrunners for the prize.
A combination of points from televoting and national juries decides the winner.
Each country awards points to 10 competitors based on judges’ scores and a public vote, with 12 points being the maximum awarded.
Earlier this week 37 countries competed in the semi-finals for 20 places, with the Eastern European candidates dominating the list of finalists.
Romania, Serbia and Italy are among those tipped for success. But they will face stiff competition from the Russian “Buranovo Grannies”.
The group comprises six pensioners from a church choir in rural Russia who will perform Party for Everyone, a cross between a traditional folk tune and a dance track.
The grannies include Natalya Pugacheva who, at the age of 77, is the oldest ever participant in the contest.
Ireland will be represented by pop duo Jedward for the second year running, following the brothers’ finish in eighth place last year.
Their performance of pop track Waterline will end with the 20-year-old Grimes twins jumping into a fountain in the middle of the stage.
The former X Factor contestants described the occasion as “awesome”.
“We’ve bought Eurovision back to the glory days,” John said.
France, Italy, Spain, Germany and the UK automatically qualified for the final as they contributed the most money to the competition. Hosts Azerbaijan also went straight through.
Britain has fared poorly in recent years. The UK has not won since 1997 and has come last three times in the past 10 years.
But there are high hopes for Engelbert Humperdinck – affectionately known as “The Hump” – given his huge global fanbase and strong following in Eastern Europe.
The veteran crooner has received four Grammy nominations and a Golden Globe for Entertainer of the Year.
He is also one of only a handful of artists with a star on both the Hollywood Walk of Fame and the Las Vegas Walk of Fame.
“I never realized the magnitude of it… how big it was,” he said.
“Now I am here I think the responsibility is pretty heavy-duty. But I am really thrilled to representing my country in this contest.
“It’s my job, I am singer and I do have 45 years of experience. So I hope it has some bearing on whatever the results will be.”
The Eurovision Song Contest is one of the most watched television events in the world, with 125 million viewers expected to tune in.
The show will kick off at 20:00 BST and finish at 23:15.
Following last year’s unprecedented Eurovision victory in Germany, the Azeri government, eager to boost the oil-rich country’s global image, has poured money into preparations for the contest.
But media attention has proved both positive and negative, with much of the focus on Azerbaijan’s poor human rights record, and protests by democracy campaigners being broken up in the capital Baku.
Eurovision Song Contest
• Created by Marcel Bezencon in 1955
• First held in Lugano, Switzerland, in 1956, when the winning song was Refrain performed by Lys Assia
• Twenty-six countries are competing in the 2012 final
• Watch clips and find out more at the Eurovision website (www.eurovision.tv)
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