G4S CEO to resign. Photo: G4S

Security group G4S has announced that CEO Nick Buckles, who presided over the firm's Olympic security shambles, is stepping down from his position at the end of May.

Buckles will be replaced by G4S's current CFO Ashley Almanza, who only joined the company three weeks ago. Shares rose 3% following the announcement.

Buckles' departure comes at the end of a turbulent 18 months for G4S. Last summer the group was excoriated for failing to provide enough security guards for the 2012 Olympic Games, a failure which forced the Government to draft in thousands of soldiers and policemen at the last minute.

Buckles was subsequently forced to appear before MPs and admit the episode was a "humiliating shambles." for his company.

The fiasco came just months after Buckles dented investor confidence by launching a £5.2bn takeover of Danish cleaning firm ISS. The deal was aborted in early 2012, prompting another public apology from the CEO.

Earlier in May, the company said its 2013 profits would be lower than previous expectations due to problems in the Netherlands and in Africa. The news sent the shares tumbling 14 percent, and analysts began to speculate about Buckles' exit from the company.

"Nick Buckles made a massive contribution to the group over a 28 year career," G4S chairman John Connolly said in a statement.

"As CEO he led G4S in the creation of significant shareholder value following the merger of Securicor and Group 4 Falck - building the world's leading security company."

In 2002 Buckles became the CEO of Securicor, which later merged with the security businesses of Group 4 Falck, creating G4S. He was appointed CEO of G4S in 2005.

Almanza will take up his new role on 1 June. The company also appointed Stuart Curl, its regional CFO for the UK, Ireland and Africa, as acting CFO.

The new role of COO will be taken over by Eddie Aston from DHL, where he is CEO of global sectors.

"G4S is a great global business with enormous long term potential and I am looking forward to working with a highly experienced and capable management team as we take the business forward. I am also delighted that we have strengthened the team with the appointment of Eddie Aston," Almanza said.