A report published last week by Amicus, the trade union that represents ESB engineers, said the fact that the two interconnectors were both fed using the same 50km stretch of pipeline and gas compressors in Scotland made them "very vulnerable to supply failure".

This newspaper has also learned that a major natural resources company, Marathon Oil, wrote to CER saying it did not regard the two interconnectors as "independent sources of supply" for security of supply purposes in 2003.

A spokeswoman for Bord Gais said the company was not concerned about the prospect of a failure at the Scottish end of the interconnector pipelines.

"A failure of the 50km single section onshore pipeline in Scotland was less of a concern, since an onshore pipeline can normally be repaired within days in most circumstances, " she said.

She said domestic and business customers would not be affected during any repair of the pipeline, although power stations would have to switch to backup fuels to preserve gas supplies.

She also said Bord Gais had decided against feeding the two interconnectors from different sources when it built the second interconnector, because "the additional capacity wasn't required at that time".

Meanwhile, a CER spokesman said that "even if there is a major rupture [of the pipeline], supplies to customers will not be interrupted". He added that Bord Gais had detailed emergency plans for dealing with such a situation.