Murder investigations have been launched after two stabbings over the weekend in London and Somerset.

A shop worker was stabbed to death in Pinner, north-west London, on Sunday morning during a robbery at a newsagent. The man, who has yet to be identified, is believed to have been opening up the newsagent on Marsh Road when he was attacked. Police were called at about 6am and the man was pronounced dead at 6.46am.

DCI Simon Stancombe said: “This was a violent robbery that has escalated, resulting in the murder of a man. It appears the till was stolen from the shop during this robbery and this may have been discarded by the suspect. If you have come across this, we want to hear from you.”

He also appealed for help tracing a black Vauxhall Corsa seen near the crime scene.

It was the 28th murder investigation launched by the Metropolitan police in the capital this year and the 29th in London overall, including one being investigated by the British Transport Police.

Nick Hurd, the MP for Ruislip, Northwood and Pinner, said he was “deeply saddened” by the stabbing.

One resident, Mary MacNamara, told the Press Association the government was too preoccupied with Brexit to tackle knife crime. “They should be looking at the causes ... It’s very infuriating to the people in this country,” she said.

In Wells, Somerset, police arrested eight people after a man in his 30s died on Saturday. Officers were called to a disturbance at a property on Merlin Drive at 4.25pm. They found two men with knife wounds on nearby Wookey Hole Road, one of whom died at the scene. A formal identification had yet to be carried out but his next of kin had been informed.

A police spokesperson said the injuries sustained by the other man, also in his 30s, were not life-threatening but he continued to receive treatment in hospital.

Armed officers assisted with the arrest of six men and two women, all from the Somerset area.

DI Alistair Hammett, of Avon and Somerset police’s major crime investigation team, said: “We’re carrying out a full investigation into what is clearly a very tragic incident. Specially trained officers will be providing support to the man’s family and our thoughts are with them as they come to terms with the devastating news they have been given.

“I appreciate this incident will shock local residents and we are increasing patrols in the area to provide reassurance. At this stage, we believe this to be an isolated incident involving people known to each other.”

A spate of fatal stabbings has sent the issue of knife crime rising up the political agenda in recent weeks, with debate over whether police have the resources to tackle the problem. In response, the chancellor, Philip Hammond, handed an extra £100m to police forces in England and Wales.

On Friday night, a boy, believed to be 17, was stabbed to death outside a block of flats in Isleworth, west London.