“Relaxing any of the measures currently in place would risk damage to both public health and the economy,” he told newsmen.
Raab is deputising while Prime Minister Boris Johnson recuperates from COVID-19 complications that nearly cost him his life.
The UK has the fifth-highest official death toll from COVID-19 in the world, after the U.S., Italy, Spain and France, though British figures only cover hospital fatalities and the real number is probably much higher.
The announcement, which had been widely expected, means Britons must stay at home unless they are shopping for basic necessities, or meeting medical needs.
Citizens are allowed to exercise in public once a day, and can travel to work if they are unable to work from home.
The measures were announced on March 23 for an initial three-week period.
The arrangements, which mirror similar restrictions in many other countries, are unprecedented in peacetime Britain and have effectively shuttered vast swathes of the world’s fifth largest economy.
Earlier, Health minister Matt Hancock warned the virus would “run rampant” if the restrictions were lifted too soon.
A YouGov poll conducted before the announcement showed 91 per cent of Britons supported a three-week extension to the lockdown.
The UK’s death toll from COVID-19 in hospitals rose 861 to 13,729, as of 1600 GMT on April 15.
Broader statistics that include deaths in care homes and in the community suggest the total toll is much larger.
Amid all the gloom, however, there was some hope.
Tom Moore, a 99-year-old British war veteran, completed 100 laps of his garden, raising over 12 million pounds (15 million dollars) for the health service.
“For all those people who are finding it difficult at the moment: the sun will shine on you again and the clouds will go away,” Moore said.
Restrictions across the globe have effectively closed down much of the world economy, and the UK is heading towards its deepest depression in three centuries.
As leaders around the world begin to contemplate ways to exit the shutdown, epidemiologists have cautioned that a second wave of the outbreak could endanger the weak and elderly.
Neil Ferguson, a professor of mathematical biology at Imperial College London who advises the government, said Britain would probably have to maintain some level of social distancing until a vaccine for the novel coronavirus is available.
“If we relax measures too much then we will see a resurgence in transmission.
“If we want to reopen schools, let people get back to work, then we need to keep the transmission down in another manner,” he told BBC radio.
On Wednesday, GlaxoSmithKline Chief Executive Officer, Emma Walmsley, said a vaccine was unlikely to be ready before the second half of 2021. (Reuters/NAN)