The number of confirmed cases of monkeypox in the UK has more than doubled to 57.
The new figure from the UK Health Security Agency is an increase on the 20 cases previously confirmed.
England now has 56 confirmed cases with Scotland confirming its first case on Monday.
Northern Ireland’s Public Health Agency and Public Health Wales each said they have had no confirmed cases.
Health officials said that while the outbreak was “significant and concerning”, the risk to the UK population remained low.
The government has stocks of the smallpox vaccine that is being offered to very close contacts of those affected.
Those at the highest risk of contracting the disease are being asked to isolate at home for 21 days, with others warned to be on the lookout for symptoms.
Transmission between people is occurring in the UK, with a large proportion of cases identifying themselves as gay or bisexual, and men who have sex with other men.
Monkeypox is not normally a sexually transmitted infection, but it can be passed on by direct contact during sex.
It can also be spread through touching clothing, bedding or towels used by someone with the monkeypox rash, and through the coughs and sneezes of someone with the infection.
The disease is usually mild but can cause severe illness in some cases.
Symptoms include fever, headache, muscle aches, backache, swollen lymph nodes, chills and exhaustion.
A rash can develop, often starting on the face, which then spreads to other parts of the body including the genitals.
Dr Susan Hopkins, chief medical adviser at the UK authority, thanked people affected for coming forward, saying they were “helping us limit the spread of this infection in the UK”.
“Because the virus spreads through close contact, we are urging everyone to be aware of any unusual rashes or lesions and to contact a sexual health service if they have any symptoms,” Dr Hopkins said.
UK Prime Minister Boris Johnson has said monkeypox is a rare disease but it is important to “keep an eye on it”.
“It’s basically a very rare disease and so far the consequences don’t seem to be very serious, but it’s important that we keep an eye on it and that’s exactly what the the new UK Health Security Agency is doing,” Mr Johnson said on a visit to a school in south-east London.
Asked whether there should be quarantine for visitors or the use of the smallpox vaccine, he said: “As things stand, the judgment is that it’s rare.
“I think we’re looking very carefully at the circumstances of transmission," Mr Johnson said. "It hasn’t yet proved fatal in any case that we know of, certainly not in this country.”
Downing Street said there were no plans to hold a meeting of the Cobra emergency committee over monkeypox, or to impose any travel bans.
Mr Johnson's official spokesman said that while vaccines were being offered to close contacts, there were no plans for an “at scale” vaccination programme.
Monkeypox is usually found in West Africa and does not often spread elsewhere.
Sir Jeremy Farrar, an expert in infectious diseases, said "super-spreader' events were likely to be behind the rise in global cases.
Monkeypox has been reported in more than a dozen countries including Spain and Portugal, say epidemiologists at Harvard University.
“The virus may have changed but I think that’s unlikely," Mr Farrar told BBC Radio 4.
"More likely is, I think, that the niche that this virus now finds itself in has allowed for some super-spreader events and those individuals involved in that have then travelled to other parts of the world and taken the infection with them."