Tookey, who is already due to leave Britian’s biggest retail bank early next year after a wholesale shake-up of top management by Horta-Osorio, will now run the group until his boss returns before the end of the year.

Since becoming CEO in March Horta-Osorio has embarked on a plan to cut some 15,000 jobs to help get crisis-hit Lloyds back on track.

“Following medical advice António Horta-Osório is taking a temporary leave of absence from his duties as group chief executive ... António is expected to return to his position before the end of the year,” the bank said.

Lloyds shares were down by 5 percent at 29 pence in late morning trade, making the stock the worst performer on Britain’s benchmark FTSE 100 index, which was down by 0.5 percent.

“With Tim Tookey also set to leave in February and no replacement as yet announced, this potentially leaves Lloyds light on senior management at a time when it is navigating through a difficult period and only soon after it announced a new strategic plan,” Shore Capital said in a research note.

One recent Portuguese newspaper interview with Horta-Osorio described the former Santander executive as “the Mourinho of the finance world,” a reference to the mercurial manager of Spanish football club Real Madrid, and told of how when he broke his right wrist he learnt to play tennis with his left.

A Lloyds employee recently said Horta-Osorio was “intense and scary,” although a trade union official said his members respected Horta-Osorio.

“I think he’s got a good reputation. He spends a lot of time visiting the branches and offices. He’s not one of those chief executives who’s holed up in Gresham Street (Lloyds’ London headquarters),” said the union member, who declined to be named.

Lloyds’ shareholders added that they supported what Horta-Osorio had done so far at Lloyds, which is some 40-percent owned by the British government following a state-led bailout during the 2008 credit crisis.

“Investors thought that Horta-Osorio had the right strategy to turn Lloyds around,” said SVM Asset Management fund manager Colin McLean.

“However, investors would be reassured if his leave proves to be only a temporary and brief one,” added McLean, whose firm holds Lloyds shares.

Horta-Osorio took over as chief executive eight months ago after joining the group from Spanish-owned rival Santander UK at the end of last year.

Since arriving at Lloyds he has quickly embarked on a restructuring program which has entailed plans to axe some 15,000 jobs, halve Lloyds’ international presence and sell off some 630 retail bank branches.

His arrival has also seen the departure of Lloyds executives who were associated with the previous management led by former CEO Eric Daniels.

In September Lloyds announced that Tookey would leave after February 2012, while former Lloyds retail banking head Helen Weir and insurance head Archie Kane have also stepped down.

In return, Horta-Osorio has brought over from Santander former colleagues such as Nathan Bostock, who is seen as a potential new finance director, dealmaker Toby Rougier and retail bank specialist Alison Brittain.

A source with knowledge of the matter told Reuters that Horta-Osorio’s workload had brought on his illness.

“He’s been suffering from fatigue due to over-work,” said the source.

Cavendish Asset Management fund manager Paul Mumford, who also holds Lloyds shares, agreed that news of Horta-Osorio’s sick leave was disappointing for shareholders.

“He’s got Lloyds back on the right track,” he said.

Horta-Osorio’s strategy update, announced in June, stated that he planned to have cut costs by some 1.5 billion pounds a year by 2014, and boost the bank’s performance by cutting through middle management to make it more agile.

Horta-Osorio aims to cut Lloyds’ international presence to fewer than 15 countries from 30 now in order to focus more on domestic retail banking, where it is market leader and has historically been far more significant than its presence overseas.

Lloyds has also said it intends to decide by the end of the year on either a sale or spin-off of the 630 branches which it has been ordered to sell by regulators as payback for a state rescue package during the credit crisis. The Portuguese-born banker’s hobbies including swimming with sharks and scuba diving as well as tennis.

A graduate of management and business from Universidade Catolica Portuguesa and with an MBA from business school INSEAD, Horta-Osorio started his career at CitiBank in Portugal, where he became head of capital market for Portugal. He worked for Goldman Sachs for two years in New York and London, focusing on corporate finance activities in Portugal.

He joined Santander in 1993, and ran operations in Portugual and Brazil before arriving in Britain, just after squeezing in an advanced management program (AMP) at Harvard, an eight-week intensive course for senior managers.

Britain ended up with stakes of around 40 percent in Lloyds and 83 percent in Royal Bank of Scotland after rescuing both during the original banking crisis with taxpayer bailouts, and RBS has also been told to sell off a host of assets.

Lloyds was one of the world’s most profitable banks in the 1990s but its growth strategy stalled after it was blocked by regulators from buying former building society Abbey National in 2001.

Instead it went on to buy troubled rival HBOS at the height of the 2008 credit crisis in a deal brokered by the Labour government of the time.

However, the acquisition gave it a legacy of bad debts that has undermined the business, and its shares have consistently traded well below the 63.1 pence average price at which the British taxpayer acquired its stake in the bank.