Experts in the power industry are warning there could be an increased risk of blackouts over the Christmas holidays.
Lack of maintenance could cause blackouts, experts say
Urgent maintenance on the UK's power network has not been completed, internal documents from the National Grid Transco show.
The documents - a series of e-mails - have been shown to the BBC.
National Grid Transco said repairs were ongoing and would be completed in the next few days.
In the e-mails, staff describe a backlog of urgent work dating from September, as well as a series of equipment and system failures.
"I don't want to spend my entire Christmas standby trying desperately to keep poorly maintained air systems together," one e-mail reads.
Another describes a potential cascade air system failure and says the system is close to breaking point.
The e-mails follow a blackout which hit London in August, resulting in Tube passengers being led through darkened tunnels to safety and traffic gridlock as lights failed.
In an earlier investigation by BBC Radio 4's Today programme, engineers blamed this on a known oil leak resulting from poor maintenance.
After seeing the e-mails, former National Grid manager David Kirkland told the BBC it appeared potentially lethal equipment was in a state of disrepair, increasing the risk of blackouts over the Christmas holidays.
Mr Kirkland, once responsible for maintenance for a quarter of the UK, said the e-mails related to dangerous high pressure.
He said the work should be done as a priority, both in terms of health and safety and because it was operationally essential.
National Grid Transco said that if a compressor, as mentioned in the e-mails, had failed, it was designed to do so in a safe manner.
The company said at no time was the security of the system jeopardised.
It said concerns about overdue maintenance were reported in September and faults reported in December.
Normal action was taken to rectify the faults and work started at the beginning of this week was expected to be completed in the next few days.
National Grid Transco has consistently denied that there is a maintenance problem.
The company described August's blackout in London as "an isolated and unusual incident" which it said had been caused by a fuse.
The Today programme also found that the company had failed to complete statutory checks on equipment.
Since privatisation in 1990 the National Grid has been reorganised three times, resulting in the loss of half of the original 7,000 staff.