Boris Johnson has said a decision on cutting Covid isolation to five days will be made “as fast as possible” after modelling found that two extra days of freedom increased infection risks by only two percentage points.

Modelling by the UK Health Security Agency (UKHSA), submitted to the Cabinet Office, suggests a five-day isolation – with release dependent on two negative lateral flow tests – would mean about eight per cent of people would still be infectious at the point of release, compared with six per cent under the current seven-day rule.

On Tuesday, The Telegraph revealed that the rules were set to change after Sajid Javid, the Health Secretary, added his voice to warnings that lengthy isolation sectors are crippling major sectors including the NHS.

On Wednesday, Mr Johnson said he was day “certainly looking” at reducing the Covid self-isolation period in England, with an announcement expected as soon as Thursday. The Prime Minister told MPs he hoped to “bring you more about that as fast as possible”.

Earlier this week, health chiefs were forced to rewrite their own advice after claiming that it was “not comparing like with like” to compare Britain’s rules with those of the United States, where isolation is five days.

One of their central arguments was that the isolation period in the US begins later in the UK because it depends on a positive test result rather than the appearance of symptoms.

On Monday, the UKHSA admitted this was factually incorrect, with both countries counting the isolation “clock” from the point at which symptoms appear unless cases are asymptomatic.

In the United States, those leaving isolation at five days are also asked to wear a mask for a further five days.