Armitt, who made the shock announcement last Friday, said he was seeking a "new challenge".
He said: "I am happy with what I achieved. I feel like I have one more decent challenge left in me." Armitt is to remain in the job until a successor has been found. The company says the search for his replacement has started.
At the time he took over at Costain in April 1997 the company had posted losses of more than £600m, accumulated throughout the early 1990s, and trading in its shares was suspended. In Armitt's first year, Costain made a pre-tax profit of £500,000. The company announced a pre-tax profit of £6.5m for the year to 31 December, 2000.
He said: "The survival part has been done, now the growth is there to be gained. The company is stable so it's a good point to leave and for someone else to come in." Armitt, who is 55, said he wanted to remain in the industry but was tight-lipped on his options.
He said: "I suspect I will stay in the industry but I've got an open mind about what that will be." Costain has been largely forgotten by the stock market because three investors hold 71% of its shares. Malaysian group Intria holds a 37.5% stake, Kuwaiti company Mohammed Al-Kharafi & Sons a 23% holding and Saudi Arabia's Raymond International owns 11%.
The share price stayed put at 10.75p after Armitt's announcement.
An analyst said: "He will be a big loss, he's done a creditable job considering what he inherited." Armitt dismissed speculation that he had left because he was frustrated by the limited shareholding and the fact that the shareholders were represented on the board.
He said: "The company has not been beholden to the City because it doesn't have much interest in us. That's given me a freedom of operation."
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