The dollar could collapse if Opec officially admits considering changing the pricing of oil into alternative currencies such as the euro, the Saudi Arabian foreign minister has warned.

Prince Saud Al-Faisal was overheard ruling out a proposal from Iran and Venezuela to discuss pricing crude in a private meeting at the oil cartel's conference.

In an embarrassing blunder at the meeting in Riyadh, ministers' microphones were not cut off during a key closed meeting, and Prince Al-Faisal was heard saying: "My feeling is that the mere mention that the Opec countries are studying the issue of the dollar is itself going to have an impact that endangers the interests of the countries. "There will be journalists who will seize on this point and we don't want the dollar to collapse instead of doing something good for Opec."

After around 40 minutes press officials cut off the feed, which had been accidentally broadcast to the press room.

Prince Al-Faisal added: "This is not new. We have done this in the past: decide to study something without putting down on paper that we are going to study it so that we avoid any implication that will bring adverse effects on our countries' finances."

Iran and Venezuela have argued that the meeting's final communique should voice concern about the level of the dollar, which has recently fallen to new record lows against the euro. They are pushing for oil to be denominated against a basket of currencies.

The greenback also weakened slightly against the pound, although sterling's own recent weakness has pushed it down from $2.10 to $2.0457 during the week.

Nigerian finance minister Shamsuddeen Usman said that Opec could declare in the communique that: "While underlining our concern for the continued depreciation of the dollar and its adverse impact on our revenues, we instruct our finance ministers to study the issue exhaustively and advise us on ways to safeguard the purchasing power of our revenues, of our members' revenues."

Chancellor Alistair Darling will today urge his fellow finance ministers at a major G20 summit to increase investment in oil production and refinement.