Prime Minister Boris Johnson and Labour leader Keir Starmer stood side-by-side to lay flowers at the scene of the fatal stabbing of MP Sir David Amess, which police believe could be linked to Islamist extremism.

Commons Speaker Sir Lindsay Hoyle and Home Secretary Priti Patel also laid a wreath at the front of Belfairs Methodist Church in Leigh-on-Sea, where Sir David was killed on Friday.

It came after Scotland Yard said the country’s most senior counter-terror officer, Deputy Assistant Commissioner Dean Haydon, has formally declared the incident as terrorism and said early investigations have revealed “a potential motivation linked to Islamist extremism”.

Sir David, 69, who had been an MP since 1983, was fatally injured while meeting constituents.

A 25-year-old man arrested at the scene on suspicion of murder is in custody at an Essex police station.

Official sources told the PA news agency the man is believed to be a British national with Somali heritage.

As part of the investigation, officers are also carrying out searches at two addresses in the London area, the Met said.

Sir David’s death has prompted police forces to contact all MPs to discuss their security and personal safety.

Ms Patel said a balance could be found between the democratic process and the security of MPs, adding that “we cannot be cowed by any individual or any motivation… to stop us from functioning”.

Speaking at Southend police station, she said: “He was killed doing a job that he loves, serving his own constituents as an elected democratic member and, of course, acts of this… are absolutely wrong and we cannot let that get in the way of our functioning democracy.”

She added she is making sure “all measures are being put in place” so MPs can continue to do their jobs safely.

Veteran Labour MP Harriet Harman said she will be writing to the Prime Minister asking him to back a Speaker’s Conference to review the safety of parliamentarians.

Tory MP Tobias Ellwood, who was hailed as a hero for his attempts to save the life of Pc Keith Palmer during the Westminster terror attack, said face-to-face meetings with MPs should be paused until a security review has been completed.

Investigators believe Sir David’s killer acted alone and are not seeking anyone else in connection with his death.

According to reports, the knifeman was waiting among a group of people to see Sir David at the church and launched the attack shortly after the MP arrived.

Local councillor John Lamb, who arrived at the scene shortly after the incident, told the Daily Mail Sir David was with two female members of staff – one from his constituency office and one from his parliamentary office – when a man “literally got a knife out and just began stabbing him”.

Chief Constable of Essex Police Ben-Julian Harrington said Southend West MP Sir David was “simply dispensing his duties when his life was horrifically cut short”.

Tory veteran Sir David, who was described by Mr Johnson as “one of the kindest, nicest, most gentle people in politics” was married with five children.

The attack came five-and-a-half years after Labour MP Jo Cox was killed by a far-right extremist in her Batley and Spen constituency in West Yorkshire.