The price rose 42p in hectic morning trading and finished at 159p, a five-year high.
Dealers suggested that Britton Group, an acquisitive plastic bag manufacturer founded in 1992 by two former corporate financiers, might be behind the approach.
Two other potential buyers, Field Group, floated last July, and Low & Bonar, the Dundee packaging firm, denied involvement.
It is estimated that a bid for the ordinary shares could be pitched at 170p, valuing the group with its convertible preference shares at pounds 100m. NMC makes plastic bags for supermarkets and owns a cardboard carton manufacturer in the US. In 1991, it made an unsuccessful pounds 32m attempt to take over its packaging rival API.
Taxable profits peaked at pounds 10.1m in the year to March 1990 but slumped to pounds 5m in 1991-92. In 1992-93 it made pounds 7.1m and in the half-year to last September pre-tax profits rose to pounds 4.3m from pounds 2.8m.
Britton has grown out of a former oil and gas exploration firm called Firstland Group. New management has increased the group's operations since 1992, making several acquisitions, including Gelpak, a plastic bag maker. Last year it raised pounds 30m to buy Taco, a producer of polythene film.
Robin Williams, chief executive, and Simon Beart, finance director, used to work for Salomon Brothers. In the first full year under new management, Britton made pre-tax profits of pounds 2.7m.
Britton shares were unchanged yesterday at 165p.Reuse content