A photograph believed to be the final one taken of PC Keith Palmer has emerged. The picture, taken by US tourist Staci Martin, was captured outside the Houses of Parliament just 45 minutes before the officer was stabbed during the Westminster terrorist attack.

Martin was in London visiting her son when she asked for a picture with PC Palmer on the afternoon of 22 March.

She told ABC News: "It's my first time in London and I see his hat and I'm like I have to take a picture of him with his hat.

"I walked up to him and said, 'do you mind if I take a picture?' He said, 'no problem', he was really nice."

Palmer was stabbed by Khalid Masood after confronting the terrorist after he crashed into the gates of the Houses of Parliament having just ploughed into pedestrians on Westminster Bridge, killing at least three people and injuring dozens more.

Hailed as a "brave and courageous" policeman, Palmer was late revealed to have been nominated for best thief taker in the Commissioner's Excellence Awards in 2015, having made more than 150 arrests in 12 months.

Met Police confirmed following his death, Palmer's shoulder number 4157 will be retired and not issued to any other officer as a mark of respect.

Elsewhere, a JustGiving fundraising page for Palmer's family has raised more than £430,000 ($537,000). Palmer leaves behind a wife and a five-year-old daughter.

A statement from Metropolitan Police Federation's Stephen Redgewell, who set up the page, said: "Big thank you to all those out there that have raised money via their own appeals and collections at work etc, and have donated it to this link or paid into the bank account for this link.

"Your efforts and contributions are greatly appreciated and will all be passed onto Keith's family."

In a statement, his family said: "Keith will be remembered as a wonderful dad and husband. A loving son, brother and uncle. A long-time supporter of Charlton FC. Dedicated to his job and proud to be a police officer, brave and courageous. A friend to everyone who knew him. He will be deeply missed. We love him so much.

"His friends and family are shocked and devastated by his loss and ask that they are left to grieve alone in peace."

Westminster attack
Cressida Dick, incoming Metropolitan Police Commissioner, stands during a minute's silence outside New Scotland Yard following the Westminster attack Neil Hall/Reuters