Luxurious Highland retreat Skibo Castle has closed the books to new members a year ahead of schedule, after enjoying a significant improvement in its operating performance.

The favoured bolthole of A-list celebrities such as Madonna has reached its target of signing up 500 individuals to Skibo's Carnegie Club, who each paid a joining fee of £23,500. They will also shell out £4700 a year just for the privilege of signing the visitor's book at the residential and sporting complex near Dornoch, Sutherland.

From March 2008, Skibo will close its door to non-members, who are currently allowed a single visit.

Skibo was owned by Peter de Savary for 13 years until 2003, when the multi-millionaire hotelier sold it for an undisclosed sum to a small group of private investors. Chosen as the venue for Madonna's wedding to film director Guy Ritchie in 2000, and patronised by film stars such as Catherine Zeta Jones, Sir Sean Connery and Ewan McGregor, Skibo has been investing heavily to help maintain its edge in the high-end market.

More than £3.7m has been ploughed into new facilities in the last two years and director Peter Crome said the benefits are starting to feed through to the bottom line.

Newly-published accounts show that Skibo boosted underlying sales by 42% in the year to March 31, to £9.1m, compared with £8m in the previous 15-month period. The company reported a small operating profit of £275,000 for the year, compared with an underlying deficit of £649,000 in the previous 15 months.

At the pre-tax level, Skibo benefited from exchange gains exceeding £500,000 in 2007, but was nevertheless carried £106,000 into the red by a £898,000 interest bill. This is a major improvement on the prior period, however, when underlying pre-tax losses totalled £1.9m.

Crome said: "We enjoyed a very good year in 2007 after investing heavily the club, which has proved to be enormously successful and popular. The directors are looking forward to continuing growth in 2007-08."

Skibo increased staff numbers to more than 200 last year, recruiting 19 extra employees to manage the golf course and 6500-acre estate. Around 100 are full-time.

Skibo was restored and rebuilt by Dunfermline-born tycoon Andrew Carnegie, who bought the castle in 1898 as a summer home.