The company, which owns Great Western Railway (GWR), said it lost £327m in the year to 31 March, compared with a profit of £152.6m last year.

The group's chairman said the results "fell short of our ambitions".

FirstGroup shares were down 10% at 99.9p and have halved in the past year.

First also runs South Western Railway, the Heathrow Airport Connect service and First Bus in the UK, as well as the Greyhound bus business in the US. It employs about 100,000 people.

Mr O'Toole said: "The time is right for me to step aside. Today's results clear the way for the new approach sought by our chairman and the board."

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Mr O'Toole, who had been with the company for eight years, has been replaced by Wolfhart Hauser, who becomes executive chairman, and Matthew Gregory, who becomes interim chief operating officer as well as chief financial officer.

Mr Hauser said: "This year's results fell short of our ambitions - we are disappointed that we did not make the further progress we intended based on the trends we saw at the end of the previous financial year."

FirstGroup was examining "all appropriate means to mobilise the considerable value inherent in the group", he added.

'Operational challenges'

As part of that process, Greyhound's business model and prospects are now under review, Mr Hauser said.

The long-distance bus service had been unable to overcome the threat posed by ultra low-cost airlines, Mr Gregory said.

In the UK, the Great Western and South Western rail franchises had "operational challenges to overcome" but were both profitable, he added.

However, the TransPennine Express franchise was loss-making and the group was providing for losses of up to £106.3m over the remaining life of the contract.

Earlier this year, FirstGroup rejected a takeover offer from US private equity firm Apollo Management, an approach that came after it warned of trading weakness due to strong competition in the US.

The company has also been targeted by Canadian activist investor West Face Capital, which disclosed a 5% stake in First last June.