Ministers are opposed to extending the boundaries of the Cairngorm National Park to embrace a further 300 square miles, for the time being at least.

They believe that any consideration of the park's boundaries should be left for the five-year review which is due to begin in 18 months.

John Swinney, the SNP MSP for North Tayside, has moved a bill to extend the park to cover the area originally proposed by Scottish Natural Heritage.

He fears that if it is left to the review, it could take until 2012 before any change could take place.

Sarah Boyack, Deputy Environment Minister, yesterday told Holyrood's environment and rural development committee that the original decision not to include Highland and East Perthshire was not "taken on a whim".

The committee was taking evidence as part of the scrutiny of Mr Swinney's bill, which would extend the boundary so that Blair Atholl and the Spittal of Glenshee would be the new gateways to the park.

It would also include the Forest of Atholl and the Beinn Udlamain mountains west of the A9.

What was Scotland's second national park, covering 1460 square miles, was created in 2003. Last week, the committee took evidence from local groups in Blair Atholl who all supported extending the boundaries. Ms Boyack reminded MSPs that the choice of boundaries was debated at length when the decision was passed in Holyrood by 100 votes to 20.

"The reasons for designation in 2002 weren't taken on a whim and it certainly wasn't a decision taken lightly and I'm very struck by having read the evidence that was given to you last week that many people were not aware in Blair Atholl of those reasons," she said.

The inclusion of the areas put forward by the new bill would raise "concern over the size of the park and its manageability by a single authority", she claimed.

The minister outlined her concerns over extending the boundaries at this time: "First, it would be damaging to the park authority's work. Secondly, it would have an impact on the national park plan.

"And thirdly, it would prevent the park's authority completing the local plan for the national park until such time the national park is actually in place and approved."

It could also cost the park authority an extra £150,000.

She praised that body for convincing local people and landowners, after initial resistance, of the benefits of creating the park. "I really don't want to disrupt and delay the good work that has been carried out thus far.

"I think the level of disappointment that would be experienced by communities and businesses in the park, who have been involved in that process in good faith, should not be ignored," she said.

With the national park plan currently with the executive for approval, any changes to the boundaries would disrupt the momentum that has been built up, she argued.

"There were good reasons for the park designation. Now is a crucial time for getting on with the national park, to move ahead and I believe there are weaknesses in the bill in front of us."

Ms Boyack claimed the bill by itself would not change the boundaries and would require amendments to the National Park (Scotland) Act 2000. Next year's planned five-year review would be the correct time to look at boundary changes.

But Mr Swinney said that the choice made by ministers over the boundary was not driven by logic.

He said: "What I've always argued is that there's a logic to having a boundary which SNH considered and recommended which effectively takes the Cairngorms massif and then applies a buffer zone about that. There needs to be a logic to the topographical judgment that is applied to bringing areas to define the boundary."