Shares of Repsol-YPF, Petrobras and BG Group rose sharply on Tuesday after Brazil’s energy regulator said that an oil field in the country to be explored by the three groups could be huge.

Haroldo Lima told reporters that the Carioca oil field “could contain reserves as large as 33bn barrels of oil equivalent” and that the find would be the world’s biggest discovery in the past 30 years.

As a result, shares of Repsol, the Spanish oil group, jumped 9.3 per cent to €25.68 and the UK’s BG Group rose 5.4 per cent. Petrobras shares were more muted, rising only 1 per cent.

The news came as oil prices reached a new record of more than $112.

Petrobras, the operator, has a 45 per cent stake in the consortium exploring part of the Carioca field, 300km off the coast of Rio de Janeiro, while BG Group owns 30 per cent and Repsol has a 25 per cent stake.

On Tuesday, however, Petrobras sought to distance itself from Mr Lima’s comments, calling them premature. Petrobras said it was drilling a second exploratory well, which was not deep enough yet to have struck oil.

“More conclusive data on the discovery’s potential will only be known after the . . . assessment process has been completed,” Petrobras said. Brazilian government officials also urged caution.

Analysts at UBS Pactual said they suspected that the 33bn boe figure might refer to early estimates for the whole Sugar Loaf area in which Carioca is situated, rather than to that field alone.

Even if the size of the discovery is confirmed, analysts noted that the oil or gas would be expensive to extract. The deep-sea field lies beneath 2,000m of water and a further 2,000m of salt rock, “making any development challenging”, Oil & Gas Daily noted in a report.

BG said on Tuesday that more evaluation work was needed on Carioca before the extent of the reserves were known. The group added, however, that it had “excellent exposure to what is turning into an exciting new hydrocarbon province”.

Offshore Brazil is a region that could hold significant potential for oil companies. In November shares in Petrobras, BG and Galp Energia temporarily soared when Petrobras said well tests on the 2006 Tupi find suggested that field could hold reserves of 5bn-8bn boe.

The Carioca discovery comes at a crucial time for Repsol, which has been struggling with declining proven oil reserves for many years. The Spanish oil group suffered a big blow last year after being expelled from Algeria.