As commuters sipped their morning lattes on London's Southbank, a small band of activists boarded their own Free Gaza "flotilla". But it was a far cry from the Mavi Marmara. The vessel was hired from London Party Boats and the downstairs bar was declared open as passengers stepped aboard. They included Green Party MP Caroline Lucas and Faithless guitarist Dave Randall. The destination was not Gaza, but Westminster, a short cruise away along the Thames.

In fact, organisers from Free Gaza and One World conceded that plans for a British boat to Gaza had all but evaporated. Instead, the supporters launched this "solidarity boat" and now plan to protest outside Downing Street.

Organiser Frank Barrett said: "As far as I know, there's no British boat going to Gaza. I don't really know what happened with it, it's very complicated."

Green Party leader Ms Lucas said she was 100 per cent behind the aims of flotilla participants. "If you have to break the law to draw attention to worse human rights abuses, then I think it is entirely legitimate," she said.

Most of the British activists who are planning to travel to Gaza have kept their identities secret, but they include Guardian journalist Ewa Jasiewicz and human rights lawyer Audrey Bomse. Also on board will be Muhammad Sawalha, a British-based Palestinian described by Iranian news channel Press TV as "the vice-chairman of the International Council for Breaking the Siege on Gaza, one of the nine organisations which spearheaded this and last year's flotilla".

Shalit family in row over letter

Activists on the flotilla this week called any requests to deliver letters to Gilad Shalit "a publicity stunt" and a "waste of time" - sentiments echoed by the captured soldier's family, but for very different reasons.

The Shalit family said that in 2010 they asked organisers to take a letter to Gaza for Sgt Shalit, who has been held hostage by Hamas in Gaza for five years.

His father Noam said their request had been refused by the activists, and they had no intention of asking them again.

Hemda Garelick, Mr Shalit's cousin, said: "The request last year was refused. The family feel that they do not want to be associated with this flotilla. They asked for a letter to be delivered last year when they believed it was a peaceful mission. This year, it doesn't look like the aim is to be peaceful."

But Freedom Flotilla II organiser Greta Berlin, co-founder of the Free Gaza movement, claimed that last year Irish Senator Mark Daly had agreed to carry a letter from the Shalit family and deliver it to United Nations officials in Gaza, but were then met with silence by the family.

"We said we would be happy to take a letter to give to John Ging, head of the UN Relief and Works Agency in Gaza. We never heard another thing from the family. Sending a letter is nothing but a publicity stunt. We were disgusted over what they did last year. I'm not going to waste my time again."

This week, Amnesty International launched a petition on behalf of Mr Shalit, calling for him to be treated more humanely and access for his family. More than 1,000 people have signed the petition so far.