A British nurse fighting for her life after contracting Ebola remains in a critical condition but she has "stabilised", the UK Health Secretary said today.

Jeremy Hunt gave an update on Pauline Cafferkey's condition as charity Save the Children launched an investigation into how she was infected - but conceded it may never establish the exact circumstances.

Mr Hunt told the Commons said he had spoken to the doctor leading Pauline Cafferkey's care at the Royal Free Hospital in north London.

He said: "As has been reported, Pauline's condition has deteriorated to a critical state although she stabilised yesterday and continues to receive the best possible care."

Mr Hunt went on: "She said in Sierra Leone that she hoped her loved ones would be proud of her. Well, she should know today the whole country is proud of her for her bravery and dedication to the service of others.

"She stands, quite simply, for the very best of NHS values."

Mrs Cafferkey, a Scottish public health nurse, had volunteered with Save the Children at the Ebola Treatment Centre in Kerry Town, Sierra Leone, before returning to the UK, and the charity said today that it was urgently reviewing its protocols.

Charity spokesman Rob MacGillivray said they had launched an "extraordinary review" to ensure that they "leave no stone unturned to, as far as possible, identify the source of this infection".

In a statement released later, Save the Children said its "serious event review" is investigating how Mrs Cafferkey contracted the disease by reviewing training, safety protocols, how protective equipment is used and "working practices".

The charity stressed that it may not be possible to be "100% sure" how the infection happened.

It said in a statement: "The early findings of the review will be made available by Save the Children as soon as possible.

"As with other Ebola infections in health facilities, it may never be possible to be 100% sure how the patient was infected.

"The work of these brave health workers is never risk-free, but we are committed to doing everything possible to learn what happened and, if necessary, to make changes to our protocols and practice.

"Staff safety is our number one priority and our thoughts are with Pauline and her family at this very difficult time."

Mrs Cafferkey's diagnosis has brought fresh scrutiny on the UK's preparations for Ebola cases.

Mr Hunt said the risk to the public remains low but stressed vigilance was required, reiterating the chief medical officer's prediction that the UK can expect to see a "handful" of Ebola cases.

He told MPs: "Both the chief medical officer and the NHS England medical director, Professor Sir Bruce Keogh, are satisfied that at this stage we have made sufficient preparations.

"However, they stress that although the risk to the public remains low, we must remain vigilant and be constantly prepared to adjust and improve our processes and protocols as this rapidly changing situation evolves."

Outlining a timeline of the handling of Ms Cafferkey's care, Mr Hunt said her temperature was tested seven times before she flew from Heathrow to Glasgow and she was cleared to travel.

She later became feverish and followed advice given to her at Heathrow to contact local services and was admitted to an isolation facility at the Brownlee unit in Gartnavel Hospital, Glasgow, at 8am on December 29.

After a blood sample tested positive for Ebola, she was transferred in a military plane to the Royal Free Hospital by 8am on December 30.

Mr Hunt said: "Some have asked if it was appropriate for her to be allowed to travel on to Glasgow after she raised concerns about her health at Heathrow.

"The clinical advice on this is clear: you can only contract Ebola by coming into contact with bodily fluids of an infected person - that means blood, vomit, diarrhoea - which becomes a risk when a patient is exhibiting feverish symptoms.

"Because she didn't have a high temperature the clinical judgment was made to allow her to continue her journey home.

"However, we also recognise that medical understanding of the disease is not complete, which is why we had already taken a number of precautionary steps that go further than strictly required by the medical evidence."

Measures include possible Ebola carriers avoiding crowded places and long journeys on public transport within the 21-day potential incubation period once they arrive back in the UK.

Mr Hunt said the guidance had been strengthened last Monday to ensure anyone from a high-risk group who feels unwell will be reassessed, advice will be immediately sought from an infectious diseases specialist and the individual will be referred for further testing "if appropriate".

He said screening arrangements at Heathrow had been found to be working well and will be kept under review. UK-based passengers on the flight Mrs Cafferkey was on from Casablanca, Morocco, to Heathrow and those on the flight from London to Glasgow have been contacted, he added.

Mr Hunt said only the US had made a larger contribution to fighting the disease in West Africa than the UK.

He added: "The safety of our volunteers is our first priority."