LONDON/CALGARY, Alberta (Reuters) - Britain's biggest oilfield, Buzzard, has been brought back to full production after a technical fault cut output by three-quarters, reducing supplies of the North Sea benchmark crude oil.

Nexen Inc, operator of Buzzard, said in a statement it resumed pumping at the normal rate of between 200,000 and 220,000 barrels per day (bpd) after maintenance issues cut output for more than a week.

Buzzard had been producing just 50,000 bpd since February 16, when Nexen found a problem in a separator unit on the offshore oilfield platform.

Crude oil from Buzzard, in the central North Sea about 63 miles northeast of Aberdeen, Scotland, feeds into Forties, one of the four key crude oil streams used to set the physical crude benchmark dated Brent.

Dated Brent is used to price oil in Europe, the Middle East and Africa, and is also the underlying market for Brent futures , which were lower around $78 per barrel on Wednesday.

"We have restored production at Buzzard to full rates," the Nexen statement said.

The Nexen statement confirmed earlier reports in the market that Buzzard output was returning to normal.

A source at a major oil company with equity crude oil from the Forties production stream told Reuters that full output was expected to be announced very soon.

"Buzzard is back," the source said.

Nexen said it planned further work on the separator unit when it shuts down the offshore field in the second quarter for one or two weeks to install facilities on a new platform.

Oil traders said on Tuesday that two of four Forties cargoes that had previously been removed from the loading programme when problems with Buzzard first appeared had since been reinstated.

This suggested problems with the Buzzard oilfield were not as serious as initially feared, they said.

Buzzard crude contains a relatively high level of the contaminant sulphur, so the sulphur content of Forties blend crude has been below normal while the field has been pumping at lower-than-usual rates, making it more valuable.