Swirls of oil in the Forth can be seen from the air

The Scottish Environment Protection Agency (Sepa) said it had been aware of the spill for two days, but the oil source had not yet been identified.

There have been no reports of dead wildlife after the incident.

However, the SSPCA warned the spill could have "catastrophic" consequences in the Forth of Forth.

A similar oil spill in a nearby section of water was reported last weekend.

It is unclear whether this latest spill is from the same source.

Our worry is that the oil is killing the food source for many of the various types of wildlife found in the area SSPCA Spokeswoman

Oil has washed up over several square kilometres on the foreshore between Longannet and Valleyfield in Fife, and as far east as the Forth bridges and South Queensferry, including Inchcolm Island.

An SSPCA spokeswoman said: "Our worry is that the oil is killing the food source for many of the various types of wildlife found in the area.

"The Firth of Forth has a very special eco-system and it doesn't take much to upset it.

"The long-term effect of any spill could be catastrophic but it may not be until the breeding season that the true extent of damage is discovered."

The spill could be seen from the Forth bridges

On Wednesday a Maritime and Coastguard Agency (MCA) plane flew over the Forth and took photographs of the affected area in an attempt to identify the source.

Graham Gordon, 25, from Edinburgh, spotted the oil spill while travelling across the rail bridge.

He said: "You could clearly see the oil lying on the surface of the water, it seems to be red or brown in colour.

"It didn't look good at all - I hope it doesn't have any adverse affect on the local wildlife."

A spokesman for Forth Ports confirmed oil had been dispersing naturally throughout the day.

Working tirelessly

He said: "As no vessels were in the area during the time period in question, the indication is that this has come from a land-based source."

Sepa's area manager Lin Bunten said: "Following initial notification on the morning of 2 July, Sepa officers have been working tirelessly day and night to identify the source of this oil.

"As yet no dead birds or fish have been identified but the spill has affected an area of several square kilometres.

"Although a conclusive source of the spill still remains to be fully identified, Sepa officers continue to work round the clock."

Dr Bob McLellan, Fife Council's depute head of pollution response, said: "We are disappointed that Forth Ports have not activated the oil spill contingency plan.