Matthew Fox is the lead actor in US drama Lost, shown on Channel 4

Twenty-three viewers complained about the frequency and placement of advert breaks after Channel 4 began screening the US series in August.

Ofcom found that Lost contained about 36 minutes of new material in its 65-minute weekly Channel 4 slot.

Channel 4 said it took a maximum number of adverts to compete with rival channels. Ofcom upheld the complaints.

'Unsurprising' impression

The watchdog found the castaway drama often began four minutes after its advertised start time and ended up to seven minutes before its official end time.

The Channel 4 programme also contained trailers and sponsor credits, in addition to three advert breaks.

"Taking all these elements, it is unsurprising that viewers had an impression of an excess of commercial material," the watchdog said.

Each Lost episode contains an average of 42 minutes of material, but up to five minutes of that comprises of credits and a reprise of events from the previous episode.

Advert gap

Ofcom found that the show did not exceed the maximum 12 minutes of advertising permitted in any one hour.

However, it also found that Channel 4 failed to comply with a broadcasting rule which requires a 20-minute gap between advert breaks.

Ofcom permits this rule to be broken occasionally to preserve a show's "natural breaks" or continuity, "but does not permit routine departure from the rule across an entire series".

Channel 4 declined to comment further on the ruling on Tuesday.