A short announcement to the Stock Exchange said: 'The company has received an approach which may or may not lead to an offer for all the shares. This approach is being considered by the board.'
Any bid would be likely to be one of the largest ever for an unquoted company. Profits in the year to March 1993 were more than pounds 50m, the last matched bargain trade on the 535(2) secondary market valued the company at pounds 650m and any bidder would have to pay a large premium for control.
A change of control would crystallise massive fortunes for the company's founders, Sir Donald and his partner, Ronald Hobson. Both have shareholdings worth more than pounds 150m.
One City trader said last night that likely bidders included large pension funds, looking to secure NCP's reliable cash generation, or property companies who would be attracted by the group's substantial town centre land holdings.
Founded shortly after the Second World War, National has grown into a hugely successful business, helped by a growing number of cars and parking restrictions.
Sir Donald last week pledged pounds 5m of his own money towards the cost of building a replacement for the Royal yacht Britannia.