The impending launch of Britain’s departure from the European Union has left campaigners against Brexit with a conundrum: fight on and seek to block the decision, or adapt and seek a new voice in the process?

For the organisers of a protest march scheduled to take place in London on 25 March, it is proving a controversial, and perhaps debilitating, choice.

“Watered-down pap” and “going mushy” are just two of the angry online responses to a rebranding exercise unveiled just days before Theresa May is due to invoke article 50 on 29 March.

In place of its original slogan “Stop Brexit”, the latest billboard campaign promoting Unite For Europe’s march on parliament this weekend has the rather less blunt catchline: “Make your voice heard”.

The role of a giant model of the prime minister with a pistol in her mouth is also now uncertain after the group’s founder Peter French – who previously arranged for the eye-catching carnival float to be imported from Germany – stepped down in mysterious circumstances.

New organisers insist their watered-down style represents a change of tactics rather than any weakening of their resolve to reverse the result of the EU referendum.

“This isn’t about moving away from our message, this is purely about branding,” said Keiran MacDermott, who was previously the group’s treasurer but is now one of three remaining committee members in charge of Saturday’s protest.

“We’ve had meetings with PR agencies and political strategists who said if we can broaden the message ever so slightly then we can still maintain our political stance but have a more family-friendly approach and this will hopefully bring up the numbers,” he added.

French had previously said he hoped hundreds of thousands of demonstrators would break UK records for a protest march on 25 March, which is also the anniversary of the European Union’s founding treaty.

But with a list of speakers still to be published and limited media coverage, MacDermott points to just 16,000 supporters saying they will go on the Facebook event page and suggests only last-minute surprise arrivals are likely to mean it exceeds that.

Neither will comment on the circumstances of the committee bust-up, which follows two other recent departures.

“People are dropping like flies to be honest. It’s a lot of work,” said MacDermott, who says he still expects to announce a full list of speakers on Tuesday, including Liberal Democrat leader Tim Farron and Labour MP David Lammy.

But the turmoil has echoes of deeper challenges facing opponents of Brexit, even while concern mounts about the perilous options facing the government once article 50 negotiations begin.

“We are NOT for soft Brexit,” said Unite for Europe’s Facebook account in a reply to some of the online criticism of its more inclusive messaging. “Unite for Europe wants to make a real difference and to do that we need to persuade people. If public opinion changes we have a chance to stop Brexit altogether – indeed that’s our only chance. If your only aim is to stick your middle finger up at leavers then by all means be disappointed. If you want to actually stop Brexit then you need to realise that antagonising the other side isn’t going to work.”

Affiliated groups called for supporters to ignore the turmoil and changing messaging and use the event to voice their own opinions about Brexit.

“To clarify, the march is unaffected and will go ahead,” said Ben Chambers of Sixteen Million Rising. “It needs maximum support from us, the boots on the ground and more than ever needs to be the biggest demonstration of opposition to a government this country has ever seen.”

Organisers stressed that the event was intended to attract people who voted leave but were having regrets, as well as core remain supporters.

“The political outcome that we would hope for is that the people of the UK have the final say on the deal and we can do that legitimately either by a second referendum or by giving our representatives a free and final say on the deal,” said MacDermott.

“Does anyone seriously think that the fact that the poster doesn’t say ‘Stop Brexit’ will persuade anyone opposed to Brexit not to turn up? ” said one online commentator, Toby Morse. “All this hair splitting is merely serving to divide what has, up to now, been a united movement. Let’s all focus on stopping Brexit rather than arguing about what should be on the poster.”

The Unite for Europe March leaves at 11am on 25 March from Park Lane in London