Campaigners against oil transfers between tankers at anchor at the mouth of the Cromarty Firth are reporting a massive increase in support for their online petition, more than quadrupling in the last week.

More than 80,000 have signed the petition calling on Chris Grayling, UK Secretary of State for Transport, to block the Cromarty Firth Port Authority's plan to transfer up to nine million tonnes of crude oil a year between tankers lying at anchor in open water, at one of the most important dolphin sites in Europe.

The petition organised by the 38 Degrees organisation, was launched eight months ago. Seven days ago around 20,000 had signed. But by yesterday afternoon that had risen to almost 81, 500. It is understood the surge in support follows it being opened to addresses across the UK rather than just Scotland, but campaigners say it underlines the national importance of the issue.

Read more: Cromarty Firth campaigners go to Holyrood

The Maritime and Coastguard Agency, which comes under Mr Grayling's Department for Transport, will have to decide whether to support an application for the oil transfers from the port authority. It has already instructed that there should be more consultation and a new application be prepared. But opposition appears to be growing.

Another petition has also been lodged in the Scottish Parliament calling on MSPs to urge the Scottish Government to ensure that environmental legislation in Scotland is sufficient to prevent ship-to-ship transfers of crude oil in environmentally sensitive locations.

To underline their argument campaign group Cromarty Rising circulated a photograph to MSPs of a humpback whale, taken by a local wildlife cruise operator at one of the proposed mooring points for the tankers.

A Cromarty Rising spokesman said "We are greatly encouraged by this surge in support. It shows the level of concern that ordinary people have throughout the UK. This has become a real people's protest and we hope the Cromarty Firth Port Authority are paying attention. It would take an incredible level of arrogance to press on with a further application in light of the growing and tangible opposition. We encourage the CFPA to drop their plans once and for all. If they fail to do so then we will redouble our efforts to ensure this plan does not go ahead".

Read more: Cromarty Firth campaigners go to Holyrood

A spokesman for the port authority said: "We believe that people have the right to protest, and it is only fair that our stakeholders are confident we look after their environment. The Port has modified its application in response to the concerns raised over the past twelve months, We fully appreciate that the environment in which we operate is extremely special, and we are legally bound to protect it."

"The Port is making an application for a licence within the law. There is a due process to follow and the relevant authorities will ultimately decide whether the ship to ship oil transfer application submitted by the Port receives approval."