Five days after the leak from the Gannet Alpha rig was spotted about 112 miles east of Aberdeen, Shell finally responded to pressure to reveal the volumes of oil involved.

It confirmed 216 tonnes had spread into the sea - the equivalent of 1,300 barrels of oil. The Scottish Government said at the weekend it involved only about 100 tonnes.

However, a raft of questions today remain unanswered, including how the leak started, why Shell has not yet been able to stop the flow of oil, where exactly the spill is in the North Sea, and whether any seabirds or other wildlife are caught up in it.

Politicians and environment groups have increased their calls for Shell to be more open about the leak. Government figures show it is four times the entire quantity of oil discharged into the North Sea in 2009, and by far the largest spill in UK waters for more than a decade.

The Scotsman's attempts to get Shell to give specific details about the leak were yesterday met by silence. And the firm refused to take part in BBC Scotland's flagship news programme Good Morning Scotland.

Environment secretary Richard Lochhead urged the company to "make information available on an open, transparent and regular basis" and said the Scottish Government was taking the spill "very seriously".

Shell, headed by chief execuive Peter Voser, insisted the leak on the flow line system that serves the Gannet Alpha platform remained "under control", and industry insiders insisted the firm would be doing everything it could to tackle the leak.