Among some groups of children in the capital, fewer than 70% have received their first dose of the MMR (Measles, Mumps and Rubella) jab, the UK Health Security Agency (UKHSA) said.

Two doses are needed to provide adequate protection, with an estimated 95% of the population required to be immunised in order to stop new outbreaks of measles, which is one of the most infectious diseases in the world.

The risk of an outbreak or measles epidemic across the UK is considered low, the UKHSA said.

However, in London, current immunisation levels suggest "between 40,000 and 160,000 cases could occur".

There have been 128 reported cases of measles in the UK from 1 January to 30 June this year - already more than double the total of 54 cases in 2022.

More than 60% of this year's cases were in London, although the disease has been detected in all regions, the UKSHA said.

Between 1 January and 20 April this year, the cases stood at 49 - meaning 79 cases have been reported in around three months.

The figures were revealed as the NHS said cases were rising across England and warned parents to ensure their children are up to date with their MMR vaccinations as the school holidays approach.

It has launched a campaign encouraging people to check their vaccination status, particularly in the capital.

Who is at risk?

The UKHSA said "susceptibility is particularly high among 19 to 25-year-olds" - whose parents may have chosen not to vaccinate them after reading widely-discredited claims the MMR jab was linked to autism by now disgraced British physician, Andrew Wakefield.

Outside London, there could be smaller outbreaks in specific groups of people including teenagers, young people and those who have not been vaccinated.

There is also a high risk of cases linked to overseas travel which could lead to outbreaks among young people and under-vaccinated communities.

The UKHSA said in a statement: "The risk in London is primarily due to low vaccination rates over several years, further impacted by the COVID-19 pandemic, particularly in some areas and groups where coverage of the first MMR dose at two years of age is as low as 69.5%."

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UKHSA consultant epidemiologist, Dr Vanessa Saliba, said: "Measles can be a serious infection that can lead to complications especially in young children and those with weakened immune systems.

"Due to longstanding sub-optimal vaccine uptake, there is now a very real risk of seeing big outbreaks in London."

Measles spreads very easily but is preventable, Dr Saliba said, meaning it is vital that people are vaccinated with two doses of the MMR jab, which is free to everyone.

It is important people are fully vaccinated before travelling overseas this summer, she added.

"Nobody wants to see their child or loved ones sick with measles, or put others who are more vulnerable, like babies, at risk.

"I urge those who have missed their MMR vaccines to catch up now."

Worldwide, measles immunisation rates nosedived in the aftermath of the pandemic, with around 40 million children across the globe missing a vaccine dose in 2021, according to the World Health Organisation (WHO).

As well as the UK, cases are rising in Russia, Austria and Serbia, the WHO warned.

In 2021, the disease claimed the lives of more than 128,000 people - mainly children under five.

Symptoms of the disease include a cough, red eyes and a facial rash.

However it can also trigger serious complications, mainly in children under five and adults aged over 30, including blindness, encephalitis and pneumonia.