Manchester Terror Attack: South Yorkshire Police officers drafted in after fatal explosion at concert

They have been deployed to Manchester along with officers from forces across the country to bolster numbers in the wake of an explosion at an Ariana Grande gig at the Manchester Arena which left 22 dead and around 59 injured.

Home Secretary Amber Rudd described the explosion as a 'barbaric attack, deliberately targeting some of the most vulnerable in our society'.

The explosion came as Britain is on a high state of alert for terror attacks following the Westminster atrocity two months ago in which Khalid Masood killed five people before he was shot dead by police.

For more than two years the official threat level for international terrorism in the UK has stood at severe - indicating that an attack is highly likely.

This is the second highest out of five levels, one below critical - which means an attack is expected imminently.

Counter-terrorism units have been making arrests on a near daily basis amid a huge effort to contain the threat to the UK, which has been underlined by a flurry of outrages in Europe.

Figures show there were 260 arrests for terrorism-related offences last year. This was a slight fall compared to the previous year, but the number remained high compared with other recent years.

Activity by police and security services has been running at a high level for a number of years as they attempt to defend Britain against dangers ranging from 'lone wolf' strikes using knives or vehicles to complex, mass-casualty plots involving guns or bombs.

In March it was revealed authorities had thwarted 13 potential attacks in less than four years, and they are carrying out more than 500 investigations at any one time.

In recent weeks there have been a number of major operations carried out by counter-terrorism teams in London.