One of Britain’s most senior former diplomats, Lord Malloch-Brown, is to take a lead role in coordinating the pro-remain groups towards a more coherent campaign focused on shifting public opinion before MPs hold a “meaningful vote” next autumn on Theresa May’s Brexit deal.

Behind the scenes discussions between the disparate anti-hard Brexit groups have led to an agreement that their messages need to be far better coordinated if public opinion is to be shifted decisively in the next nine months.

Malloch-Brown, a former UN deputy secretary general, political communications consultant and Foreign Office minister under Gordon Brown, said: “We don’t necessarily need a single organisation, but we do want everyone on the same page.

“The aim will be to shift public opinion by the time MPs come next autumn to have the meaningful vote that was agreed last week. We cannot know precisely the Brexit deal that the meaningful vote will be on, but it will be the moment to stop the trainwreck.

“There will not necessarily be a big bang launch, but the new year is likely to see a much more coordinated campaign and a more coherent, consistent message. It will be both more pocketbook and more emotional, looking at issues like the risk to the NHS.

“We need to sway public opinion nationally so that there is a majority to remain at the time of the vote in parliament. We also have to lobby in constituencies in a targeted way so we are reaching leave-voting MPs in constituencies where the majority voted remain, and we have to work in constituencies where remain MPs have been cowed by the support for leave in their seats.”

But he claimed: “The general economic concerns, the higher inflation, the more expensive holidays, the slowing down of UK investment is beginning to seep through, even if it has not yet changed the headline voting on a referendum.”

Most polls show only a small shift towards remain since the 2016 vote, although the poll outcomes are dependent on how the question is phrased.

Malloch-Brown said public opinion had not yet shifted for two reasons: “People say ‘we made a decision and there is no reason to repeat it’. By British standards, it was quite a heated and divisive debate in the workplaces and at kitchen tables of the country, and people do not want to repeat it.

“The general economic news, the risk to the health service, the levels of growth, and everything else that is important to daily lives is in jeopardy. Collectively that will lead to more people wanting to reconsider. There has never been a more gross example of mis-selling than the leave campaign.”

At present, the pro-remain campaigning is split between Open Britain, a business-oriented group backing MPs who oppose hard Brexit in parliament; Best for Britain, a grassroots-focused campaign; and the European Movement, a longstanding pro-EU network currently headed by the former Conservative cabinet minister Stephen Dorrell. Malloch-Brown has recently become chairman of Best for Britain.

One of the key advocates of greater coordination has been the former deputy prime minister Nick Clegg. Last week he warned: “It is frankly a gift to the Brexiters the way that so much anti-Brexit energy is being dissipated in so many disorganised ways.

“For the electorate to hear a message they need to hear it consistently articulated to them in a sustained way by lots of people over a long period of time. At the moment different campaign groups are coming up with their own different reasons for why they don’t like Brexit. The blunt truth is that when we have so little time, a cacophony of different messages is going to cancel each other out – which is one of the reasons why we are not making greater impact on public opinion.”

He said if “a big tent movement” could be created “that speaks with one voice, does the necessary research to know exactly what messages need to be conveyed in a consistent way, then we might just have a chance to shift the dial in the next few months. That is what is needed and time is tremendously short”.

He insisted that “we have a matter of weeks or months to bring this matter to a head when this deal, in whatever form, comes back to the Commons and Lords in a year’s time”.

Quick Guide What are Brexit options now? Four scenarios Show Staying in the single market and customs union The UK could sign up to all the EU’s rules and regulations, staying in the single market – which provides free movement of goods, services and people – and the customs union, in which EU members agree tariffs on external states. Freedom of movement would continue and the UK would keep paying into the Brussels pot. We would continue to have unfettered access to EU trade, but the pledge to “take back control” of laws, borders and money would not have been fulfilled. This is an unlikely outcome and one that may be possible only by reversing the Brexit decision, after a second referendum or election. The Norway model Britain could follow Norway, which is in the single market, is subject to freedom of movement rules and pays a fee to Brussels – but is outside the customs union. That combination would tie Britain to EU regulations but allow it to sign trade deals of its own. A “Norway-minus” deal is more likely. That would see the UK leave the single market and customs union and end free movement of people. But Britain would align its rules and regulations with Brussels, hoping this would allow a greater degree of market access. The UK would still be subject to EU rules. The Canada deal A comprehensive trade deal like the one handed to Canada would help British traders, as it would lower or eliminate tariffs. But there would be little on offer for the UK services industry. It is a bad outcome for financial services. Such a deal would leave Britain free to diverge from EU rules and regulations but that in turn would lead to border checks and the rise of other “non-tariff barriers” to trade. It would leave Britain free to forge new trade deals with other nations. Many in Brussels see this as a likely outcome, based on Theresa May’s direction so far. No deal Britain leaves with no trade deal, meaning that all trade is governed by World Trade Organization rules. Tariffs would be high, queues at the border long and the Irish border issue severe. In the short term, British aircraft might be unable to fly to some European destinations. The UK would quickly need to establish bilateral agreements to deal with the consequences, but the country would be free to take whatever future direction it wishes. It may need to deregulate to attract international business – a very different future and a lot of disruption.

Malloch-Brown said: “This is very much about trying to get a better uniform, coordinated message in the new year rather than necessarily forming one organisation – a merger could take up a lot time and effort. Perhaps there are advantages to each organisation having their own specialism.”

Pro-Europeans admit privately they are facing four big stumbling campaign blocks: the lack of a single consistent messenger persuasive with working-class and older voters, the ambivalence of the Labour leadership on Brexit, voters’ belief that the referendum is irrevocable, and finding a credible way for Conservative MPs to reject a Brexit deal in the autumn without provoking a general election.