Minke whales are among the species which have been spotted

The species included Minke, Pilot and Orca whales and bottle-nosed dolphins.

Green MSPs, who ran a sightings hotline in October and November last year, say there is inadequate information about the number of cetaceans in the Forth.

They hope to persuade ministers to intervene and prevent Forth Ports giving the scheme the green light.

Sunderland firm Melbourne Marine Services Ltd wants to transfer oil from Russia to other vessels in the Firth of Forth.

Forth Ports' contingency plans for any accidents have been approved by the Maritime and Coastguard Agency.

However, there has been opposition from local councils and environmental campaigners.

Green MSPs said Forth Ports could be blocked from approving the scheme if the Scottish Executive could show that it may endanger protected species and their habitats.

'Tip of the iceberg'

Mark Ruskell MSP, Green MSP for Mid-Scotland and Fife said: "The number of firm sightings show an impressive diversity and number of important species in the Firth of Forth.

"I am sure this is just the tip of the iceberg in the Forth and it is clear that there are significant populations of these mammals present.

Ship-to-ship oil transfers already take place at Scapa Flow in Orkney

"What we need now is political will by ministers to step in and block the oil transfer project.

"It is not only the species and ecology of the firth that will be at risk if the project gets the go-ahead but the economy around the area, which is dependent to a large degree on the state of the natural environment."

A Scottish Executive spokesman said: "The executive's interest in ship-to-ship oil transfers relates to our responsibilities for environmental protection, particularly in respect of our responsibility for the application of the appropriate EC directives for wild birds and habitats.

"We have encouraged Forth Ports throughout to engage in dialogue with Scottish Natural Heritage on the natural heritage issues highlighted in this case."

Environmental considerations

A spokesman for Forth Ports said: "As the statutory harbour authority for the Firth of Forth we are undertaking an assessment of the proposed ship-to-ship operations taking into account all safety and environmental considerations.

"In order to make an informed decision as to whether the operations should proceed we are drawing on a range of assessment reports and expert opinion.

"We are also carrying out an appropriate assessment under the Habitats Regulations in relation to the effect of ship-to-ship operations on protected species (including bottlenose dolphins) in the Firth of Forth.

"As the statutory harbour authority for the Firth of Forth we would not allow this operation to go ahead if it were likely to have a significant effect on the integrity of protected sites in the Firth of Forth."