The transaction, which is expected to be fully completed in the second quarter of the year, will see the London-based global operator continue to manage the property for at least 30 years.
The management contract also contains three 10-year extension rights which can be taken up at IHG's discretion meaning the company is likely to be in place for 60 years.
IHG first revealed it was planning to sell the 447-bedroom venue in August last year as part of its asset-light business model.
"The transaction we have announced today to sell InterContinental London Park Lane highlights the value of our asset portfolio and the attractiveness of InterContinental as one of the world’s leading luxury hotel brands," said Richard Solomons, chief executive of IHG.
The final sale figure, in excess of £300m, is 62 per cent above the hotel's net book value which was worked out in December. The cash proceeds which have been generated from the sale of the leasehold will be used for 'general corporate purposes' such as providing security over pension liabilities.
"It is another step in our long standing commitment to reduce the capital intensity of IHG," explained Solomons. "We are very pleased to be working closely with Constellation Hotels, a respected hotel investor, who will be a great partner and with whom we look forward to building a long-term relationship."
Since becoming a standalone company in 2003, IHG has sold 191 hotels. The sale of Park Lane will generate pre-tax profits for the group of around $150m.
Until last year, the InterContinental London Park Lane hotel was IHG's only London venue under the company's flagship InterContinental Hotels & Resorts brand.
The iconic property, which was first opened in 1975 and is built on the site of the Queen's childhood residence, was joined in November 2012 by the InterContinental London Westminster hotel.
In February, IHG, which also operates the Crowne Plaza, Hotel Indigo and Staybridge Suites brands, said 2012 was a year of 'significant progress' for the company which reported a global rise in revenue per available room (RevPAR) and an 11 per cent rise in profits.
When the sale of Park Lane was first mooted by the firm, IHG said it would focus its future European growth strategy on its Holiday Inn and Holiday Inn Express brands.