The company said: "The inquiry is extremely preliminary in nature and there can be absolutely no certainty that any approach or offer will follow."
City analysts speculated that the interest could have come from a rival rail contracting specialist like Amey, Balfour Beatty, Carillion or Babcock.
The shares, which have mostly limped along at below 20p since a profits warning in November 2007 wiped 75 per cent off from their value, surged by nearly a third from 17ВЅp to 23Вѕp.
Mr Norris, a former Conservative MP and twice candidate for Mayor of London, was named as chairman of the company in 2003 while it was facing a public inquiry into its role in the Potters Bar rail crash in Hertfordshire.ADVERTISEMENT
The business staved off collapse the following year by selling its ВЈ150m stake in the TubeLines underground consortium to Amey, a subsidiary of Ferrovial, the Spanish infrastructure group.
The company has had occasional interest from potential buyers over the past year.
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