The Scottish Government and islands council issued the warning after the cost of laying a new interconnector cable to the mainland to carry the extra energy generated by wind, wave and tidal energy projects increased by 65%.
The cost increased to £775m from its original projection of £490m. The estimated completion date is now October 2016 or later.
Scottish Hydro Electric Transmission, the Scottish and Southern Energy subsidiary responsible, revealed the additional costs and delays on its website. It said advanced detailed contractual negotiations with the supplier of the interconnector had concluded in late summer "...and it became clear that the cost and delivery programme agreed in October 2010 would be substantially altered". The cost is now estimated to be no less than £700m plus at least £75m on Lewis.
Western Isles Council leader Angus Campbell said it was angry with the delays, adding: "This is really going to damage confidence in the Outer Hebrides."
A Government spokesman said: "We are very concerned that the slippage exposes the project to further cost rises."