The investment is being made by JNR Eastern Investment (which stands for Jacob and Nathaniel Rothschild), the Rothschilds’ Eastern European investment company.

JNR and Vanco have pledged to invest a minimum of £60 million in exploration, although Ukraine says that these costs are likely to be nearer £400 million — and if large oil or gas deposits are found, investment costs could reach as much as £1.5 billion.

The area that the consortium will explore requires deep-sea drilling, sometimes at depths of up to two kilometres (1¼ miles). It has not been explored until now because deep-sea drilling technology has been developed only recently.

A Vanco source in Kiev said: “The key to deep-sea drilling is to be sure of your geology. We bought a seismic [survey] of the area, and we’re very confident there are deposit structures there, possibly as big as structures in the Caspian Sea. This is potentially a huge project.”

Vanco and the Rothschilds beat many bigger oil companies to win the tender, including ExxonMobil and Shell.

The Black Sea project is one of a number of efforts by Ukraine to become more independent of Russian gas. Gazprom, the Russian oil and gas giant, doubled the price that Ukraine pays for gas in January and is threatening to double it again in July. A government spokesman said that that Gazprom-owned companies had looked to buy the rights to Black Sea drilling “in order to bar Ukraine from an alternative source of oil and gas”.

Vanco sources said that, if any oil was found, a lot of it would go to supply Ukraine’s domestic energy needs.