The discovery of an explosive device outside the Dublin offices of Shell Ireland have raised concerns over a three-year controversy that has delayed the laying of a 60-mile pipeline in the Corrib gas field off Mayo.

The device, rendered harmless by Irish Army bomb disposal experts on Monday night of this week, was planted as pro-environment campaigner Maura Harrington reached the end of her first week on hunger strike outside Shell's compound at Glengad, north Mayo.

Harrington's supporters distanced themselves from the Dublin home-made bomb and insisted their campaign was based entirely on peaceful protest.

But a Shell spokesperson said the bomb, described by Gardai (police) as "viable," was a "sinister" development.

Harrington marked her first week on hunger strike - most of it spent in her locked car outside the compound gates - by resigning on her 56th birthday as head teacher at a nearby school.

She said she intends to continue refusing food until the ship hired by Shell, the Solitaire - at 310 yards long, the largest pipe-laying vessel in the world - has departed from Irish territorial waters.

The ship has been anchored for 10 weeks off the fishing port of Killybegs in Co. Donegal while protesters gathered in Mayo.

After waiting many weeks for a suitable weather window, the Solitaire sailed into Broadhaven Bay in Co. Mayo last week to lay pipes between the Corrib gas field and a refinery site on shire at Bellanaboy.

But within 24 hours it returned to Killybegs after sustaining accidental damage. A section of its pipe-laying apparatus, known as a stinger, became detached.

A spokesperson for Shell said the ship would remain in Killybegs while the damage was being assessed and any decision on where it subsequently goes will be based on that assessment.

Across in Mayo, mother-of-two Harrington and her fellow-campaigners in the Shell to Sea organization were daily being joined by more supporters.

British supporters marched in London in solidarity with her. They demonstrated outside the London offices of the owners of the Solitaire, and then marched to the Irish Embassy.

Work on the Corrib Gas field pipeline has been delayed for three years during which protesters have clashed with Gardai (police) several times.

Shell to Sea is against any pipe-lying being done on land. The organization has claimed that with planning permission for the onshore section still under consideration, work on the offshore section is "extremely premature."

They have demanded that the Green Party's two Cabinet ministers, John Gormley and Eamon Ryan, halt the offshore work.