It is their fourth protest in three days, as they continue to haunt the Japanese emperor during his visit.

The POWs are calling on the UK prime minister to have the "courage" to support their fight to win compensation and an apology from Japan.

Emperor Akihito is taking lunch with Mr Blair, his wife Cherie and other guests.

The emperor began his list of engagements on Thursday with a visit to the Royal Botanic Gardens at Kew, where he was guided around the Japanese area.

After lunch, the emperor and empress move on to a medal presentation at the Royal Society for Advancement of Science in Carlton House Terrace.

In the evening, the emperor and empress will host a return banquet for the Queen at the Victoria and Albert Museum.

As with the first two days of this visit, small demonstrations by former prisoners of war are expected to continue.

Protesters outside the Guildhall calling for compensation payments

They are demanding a formal apology and compensation from Tokyo for the torture they endured and want compensation for their suffering.

Martyn Day, lawyer for the POWs and internees associations, said: "The purpose of a letter is to ask for a meeting.

"We have asked and asked and asked Mr Blair for a meeting, but we haven't got anywhere."

Arthur Titherington, chairman of the Japanese Labour Camps Survivors Association, welcomed the continuation of the demonstrations.

"It brings it home that this issue will not go away until our government and the Japanese authorities deal with it properly."