Experts yesterday began assessing the environmental impact of the Firth of Forth sewage spill after engineers finally brought a halt to the leak.

The discharge of millions of litres of raw sewage which began on Friday was closed off only at lunchtime yesterday when engineers fitted temporary pumps at the Seafield wastewater treatment plant in Edinburgh.

Following an initial investigation, a broken bearing was last night being blamed for the malfunction of pumping equipment.

Scottish Water, for whom the plant is run by Thames Water, described the incident as a "catastrophic failure" and has apologised to customers.

The spillage has led to health fears among Edinburgh residents. Conservation groups are assessing the impact on wildlife.

Soon after the spillage was stopped, scientists from the Scottish Environment Protection Agency (Sepa) began taking samples from water in the firth and nearby shorelines.

The first tests were at south Forth estuary, including Cramond, Portobello Central and West, Fisherrow West, Seton Sands and Longniddry.

They will also take samples offshore and on the northern Fife side of the firth, with preliminary results expected today.

Colin Bayes, director of environmental protection and improvement at Sepa, said: "We have begun the next phase of our response to the Seafield sewage incident, now that the temporary engineering works to divert the discharge back into the plant have been completed.

"We think Fife is less likely to be affected by the incident, which is why the south estuary is our current priority, but we are extending our monitoring programme to the north side."

The pump failure meant that at one stage 1000 litres of raw sewage were pouring into the Forth every second.

Seafield treats sewage for 800,000 people in and around Edinburgh.

Yesterday the Food Standards Agency Scotland warned fish caught in nearby waters may be contaminated and should not be eaten, An FSAS said: "People should not eat food caught in the affected area and food should not be harvested or fished in this vicinity."

Bertie Armstrong, chief executive of the Scottish Fishermen's Federation, said: "The warning, in our view, is premature because it depends on where the stuff has gone.

"There will be very little fish caught over the weekend, and what is on the shelves now won't have been caught then.

"We should have a balanced and realistic view about the damage or lack of damage which has been caused by the discharge."

Environmental health officers from Edinburgh City Council also advised people not pick up or eat any fish or shellfish.

Calum Duncan of the Marine Conservation Society questioned the lack of back-up and added: "Given the choice of releasing the sewage to sea, or allowing it to backflow into people's homes, the correct decision was, of course, to allow it to be released to sea.

"We support calls for an investigation into this matter."

The MCS also said that some species of filter-feeding shellfish, such as surf clams and razor shells, harvested for personal consumption should not be eaten.

It was deemed too early by MCS to establish whether sea life has been harmed by the sewage, but Scottish Water chief executive Jon Hargreaves said the incident was unlikely to have caused any long-term environmental damage.

Rob Kirkwood of Leith Links Residents' Association, which is campaigning against problems at the plant, said: "The primary treatment plant is medieval, the worst performing sewage plant in the Western world, with a modern secondary plant clumsily added on to it."

Thames Water said yesterday that the 30-year-old pump was refurbished six months ago. A spokesman said it was unlikely the bearing failure could have been predicted.

The spokesman said: "The type of pump is one which is widely used throughout the global water industry and has been particularly reliable and very good at the job it is designed to do.

"Unfortunately no engineering asset is 100% failure proof."

ANALYSIS: Douglas Fraser

Website: Scottish Water

Website: SEPA