The NSPCC has warned that tech companies are enabling the crimes by failing to design their sites with children's safety in mind.
A freedom of information request by the charity revealed that 389 grooming and online offences involving a victim under-18 were recorded by West Mercia Police in 2019-20.
This was a 104 per cent increase from the previous year, and meant 17 per cent of all sex crimes recorded against children by the force were online.
In 2018-19, eight per cent of child sex crimes in West Mercia had an online element.
The number of online sex crimes against children across England and Wales topped 10,000 in 2019-20 – the 10,058 recorded was a 17 per cent rise on the previous year.
The Home Office said around 700 people were being arrested across the country each month.
The Government says its Online Harms White Paper sets out plans for world-leading legislation to keep children safe online.
Andy Burrows, head of child safety online policy at the NSPCC, said: “Offenders are using the web to commit child sex offences in ever-growing numbers and young people are at even greater risk of grooming and abuse due to the Coronavirus pandemic.
“But these crimes have been enabled by tech companies that continue to fail to design their sites with children’s safety in mind.
“The Government have a pivotal opportunity to change this in the coming weeks in their response to the Online Harms White Paper.
"By setting out bold and ambitious legislation that puts a duty of care on tech companies to protect children online, and giving a regulator the power to enforce this with financial and criminal sanctions, they can set a global precedent for preventing avoidable harm.”
The Government said it had invested heavily in law enforcement, including hosting a Hidden Harms Summit, convening a global conference to drive the response to online child sex crimes and giving £1.6 million towards the NSPCC's helpline.
A Home Office spokesman said: "Tackling online child abuse is a priority and we are working at pace to develop legislation to make the UK the safest place in the world to be online.
"This will introduce a duty of care on companies, who will need to put in place systems to deal with harmful content and take robust action, and will be overseen by an independent regulator."