Staff at large chains including Greene King and JD Wetherspoon were among those who did not recognise the code word.

The scheme, launched in 2016 and named after Angela Crompton who was killed by her husband, allows people to discreetly signal they need help by using a code word at participating venues.

The scheme enables people to signal for help at the bar by using the code word "Angela".

In total, 13 of the 25 venues visited failed to respond appropriately to the Angela code word.

But secret filming by BBC researchers found that in half of the London venues they visited, including major chains, staff failed to respond to the code word.