Shares of Malaysia's AirAsia Group and its unit AirAsia X fell on Monday, after allegations by the United Kingdom's Serious Fraud Office that Airbus paid a bribe of $50m to win plane orders from Asia's largest budget airline group.

AirAsia shares fell as much as 11 percent to 1.27 Malaysian ringgit ($0.31) - their lowest since May 2016 - while those of AirAsia X tanked 12 percent to an all-time low of 11.5 Malaysian cents ($0.03).

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Malaysia's anti-corruption agency said in a statement on Sunday that it was investigating the allegations from the UK.

In a separate case, Sri Lanka also said it has ordered an investigation into bribery allegations involving Airbus.

Documents filed in the Airbus case said the planemaker paid $50m in sponsorships to a sports team jointly owned by two AirAsia executives in return for an order for 180 aircraft, later amended to 135 planes. The executives were not named.

In one 2013 email discussing the payments, one senior Airbus employee said to another: "We need to get this done. If not we don't have a 25 A330 deal," referring to one of the company's wide-body models.

AirAsia said it never made any purchase decisions that were premised on Airbus sponsorship and that it would fully cooperate with the Malaysian Anti-Corruption Commission (MACC).

The airline defended its executives, saying they "negotiated rigorously in the interests of the company and had at all times acted in good faith".

"The entering into of each aircraft purchase agreement was never made by any single individual decision, but instead arrived at through careful evaluation, deliberation and the collective decision of the board members after taking into account technical specifications, aircraft flight performance and operating economics," AirAsia said in a statement on Sunday.

"As AirAsia and its executives have no visibility on Airbus' internal processes, we cannot comment on or be associated with any alleged failures or lapses on the part of Airbus to comply with its own policies or applicable legal requirements," the airline added.

Malaysia's Securities Commission said on Sunday it would also examine whether AirAsia broke securities laws.

The allegations were revealed on Friday as part of a record $4bn settlement Airbus agreed to with France, the UK and the United States. Prosecutors said the company had bribed public officials and hidden payments as part of a pattern of worldwide corruption.

Airbus said over the weekend it would not comment on the Malaysian investigations.

Analysts said the accusation against AirAsia comes at a particularly bad time as airlines grapple with a slowdown in business because of the fast-spreading coronavirus epidemic that has killed more than 300 people in China and disrupted air travel.

"Besides being embroiled in this corruption scandal, we expect a tough operating environment to persist over the medium term with maintenance cost remaining high ... and concerns over the Wuhan virus outbreak which could derail propensity for air travel in the region," Malaysia's Kenanga Investment Bank wrote in a research note.

TA Securities, another Malaysian investment firm, downgraded AirAsia Group stock to "sell" from "buy".

"We choose the 'sell first, ask questions later' approach to avoid the uncertainty in association with the corruption investigation by MACC, where the impact on AirAsia could be significant in terms of corporate governance," it said in a note.

In the Sri Lankan case, its government said it will conduct "a comprehensive investigation into reports of allegations over financial irregularities", according to a brief statement by the office of President Gotabaya Rajapaksa.

Detailed findings from the UK's SFO said Airbus had hired the wife of a Sri Lankan Airlines executive as its intermediary and misled export credit agency UKEF over her name and gender while paying $2m to her company.

In a statement, SriLankan Airlines said its chairman and board had directed the management to cooperate fully with any government agency regarding any investigation or prosecution.

The board had also told the management to "preserve and study all available internal documentation with a view to take all possible corrective future action," it added in Sunday's statement.

The alleged corruption in dealings between Airbus and SriLankan Airlines took place between July 2011 and June 2015, the SFO added.

Saturday's announcement of the Airbus settlement followed a nearly four-year investigation spanning sales to more than a dozen overseas markets.