Gherkin, interior

if Foster and Partners' newly occupied Swiss Re tower was any more revolutionary it would rotate. Its bulging cigar shape has ushered in a generation of office buildings with freeform curvilinear shapes. And although disguised by its slick, high-tech skin of clear glazed panels, this is London's first green skyscraper, with windows that open right up to the 32nd floor and and services that use 40% less energy than would be expected.

Yet venture into and up the 41-storey office tower, and there's something disconcertingly similar to the 1970s NatWest Tower, now known as Tower 42, which resides close by. On a typical office floor, you find yourself in a drum-shaped column-free space that is encircled by perimeter glazing and which has much of the interior taken up by a core of lifts, service risers and structural supports.

Being a Foster and Partners creation, there are, of course, several twists. The circular floors are larger than in Tower 42 and range from 26.5 m in diameter near the pinnacle to 56.5 m nearly half way up, where the bulges is greatest. The columns and glazing mullions encircling it are not vertical but form a diagonal grid. The curving perimeter wall slopes slightly outwards or inwards depending on how high up the building you are. And six sizable triangular holes have been chomped out of the edge of each floor plate, although within the glazed envelope.

It is these triangular holes that are the key to the building's interior. They have a radical effect on the building's geometry, increase daylight penetration and improve the way the internal space is used. And perhaps most importantly, they channel fresh air from the openable windows through the office areas.

On each floor, the triangular holes help transform what is a large amorphous drum-shaped space into six distinct office spaces of manageable size and shape. As Robert Harrison, associate partner at Foster, explains, each office space is more or less rectangular in shape and of constant 16.5 m width, though its depth varies from 6 m on a narrow upper floor to 15 m on a bulging middle floor. Each space can accommodate a self-sufficient group of 30 staff.

The six triangular holes link up with the holes on the floors directly below and above, although they are separated on every sixth floor by a fire-compartment floor to form a network of six-storey mini-atriums all round the building perimeter. And because structural columns at the perimeter are arranged on a diagonal grid, the holes do not line up vertically but are displaced sideways by a constant 5° on each floor so that they step up the building like spiral staircases or helter-skelters.

The six-storey atriums are known as lightwells by the architect, as they are filled with daylight and penetrate deep into the building. On each floor, they are open to the office space along one side and lined with a clear-glazed partition along the other. For an office worker sitting at a desk deep inside the building, the effect is like being surrounded by external glazed walls on three sides. In floors two to 15, which have been fitted out as the reinsurance company's London head office by the interior design division of TP Bennett, the open edges of the six lightwells on each floor have been designated as break-out spaces or balconies. Here office workers can recharge their batteries, assisted by an exhilarating sense of spiralling space and daylight. And as a unique bonus in a high-rise building, they are offered views down through lower floors of the lightwell and its glazed wall to street level, making a reassuring visual connection with the surrounding city.

The one missed opportunity in these spiral lightwells is that they have not turned out to be "gardens in the sky", as Foster had intended. Instead TP Bennett has created office interiors that have a neat, clean-cut character that matches Swiss Re's ethos, with the carpeted base of each lightwell serving as an informal meeting area for staff. Yet if the lightwells had been allowed to fulfill their role as spiralling wintergardens with lush semi-tropical greenery and hanging gardens draped over the balconies, they would have enlivened the otherwise rather bland interiors. They would also advertise the building's green credentials, not just to occupants but to all Londoners glimpsing the building from outside.

As you'd guess, the interiors are most spectacular at the topmost pinnacle, 41 storeys above the ground. Far from reserving the top of the tower for an opulent boardroom for City fat cats, Swiss Re took the egalitarian approach of designating it as an everyday restaurant and bar for all staff. Foster responded by pinching in the top of the tower to a narrow point. If this suggests a cramped attic chamber, however, the reality is just the opposite: it is a huge, lofty dome, uncluttered by internal walls or columns – breathtakingly exhilarating. And as the dome is fully glazed, the heavy diagonal frame having been discarded up here, the interior is radiant with daylight and offers a mesmerising panorama all around greater London and the green hills beyond.

As for the building's external appearance, it contrasts markedly with the Lloyd's Building, which stands just a stone's throw away, and which was designed by Norman Foster's former partner, Richard Rogers. Whereas the Lloyd's Building is a frenetic melange of projecting lifts, ducts and structural columns, the Swiss Re tower is graced by the purest building form in the City. That is partly because the circular geometry is absolutely regular from the pavement to the uppermost peak, and partly because the entire building is sheathed in a sheer glass skin with no projections, other than the opening windows. The curving shape also reduces reflections, increases transparency and makes the huge form appear slender. The paradoxical result is that whereas the tower has such an iconic curvaceous shape when seen from afar, its smooth form presents an undemonstrative backdrop to the older, more elaborate stone buildings that cluster around its base.

The bulging form brings another double benefit. By gradually expanding after it it was built, it adds floorspace without interfering with its neighbours. And by tucking inwards towards ground level, it creates a generous public plaza in front the building, which flows into the existing plaza around the Commercial Union Tower just over the road.

Again in contrast to the adventure playground of recesses, ramps and basement niches around the Lloyd's Building, the Swiss Re cylinder rises cleanly out of the pavement. The only external modelling is created by a recessed main entrance, where the diagonal structural columns are expressed as a large star-shaped sculpture rising up in front of it.

The towering presence and unmistakable curvilinear form of the Swiss Re skyscraper at last puts the stamp of Britain's most successful architect on the nucleus of Britain's business world. The tower's distinctive form, clean lines and low energy consumption also set a suitably high standard for other office developers and architects to emulate.