Labour’s John McDonnell will join thousands of protesters from across the country in Westminster on Saturday for a second referendum rally as Michael Heseltine urged Tory MPs to reject Boris Johnson’s Brexit deal.

The shadow chancellor will give a speech at the Final Say rally in Parliament Square alongside the actor Sir Patrick Stewart and the co-presenter of The Great British Bake Off Sandi Toksvig.

McDonnell is one of the highest-profile Labour figures to attend a rally organised by the People’s Vote campaign so far and comes after he vowed to personally campaign for remain should there be another poll.

But his attendance comes as Labour is believed to be cooling towards tabling a second referendum amendment of any kind to Johnson’s emergency motion on the new Brexit deal.

Before the rally, McDonnell said: “I’m proud to stand with hundreds of thousands of people in demanding that they, not wealthy, privileged and out-of-touch hard-right Tory MPs, have the final say on what is an even worse deal than Theresa May’s.

“Our policy is clear: let the people decide.”

Meanwhile Heseltine, the former deputy PM, has written to every Conservative MP urging them not to approve Johnson’s Brexit deal.

The supporter of the People’s Vote campaign said Tories should not be tempted to back it, even if they feel like “just getting it over with”.

The politician lost the Tory party whip earlier this year after he said he would vote for the Liberal Democrats in the 2019 European election in protest at his own party’s Brexit position.

In his letter, he wrote: “I do not call on any Conservative colleague to defy the whip lightly … Loyalty to the party whip is the traditional way for Conservatives to behave but defying it in moments of supreme crisis to defend the interests of our nation is the greatest of Conservative traditions.

“I understand the temptation to just get this over with, supposedly to ‘get Brexit done’. But this would not get Brexit done. It will, instead, be the start of a long and bitter series of negotiations for our country which could still end in a no deal outcome.”

The former prime ministers Tony Blair and Sir John Major have also united to make a short film warning against Brexit and its impact on Northern Ireland to be broadcast at the rally.

The pair, who faced each other across the dispatch box between 1994 and 1997 and helped broker the Good Friday agreement, have both been vociferous in their calls to avoid a hard Brexit.

Blair said: “The Good Friday agreement was a careful, painstaking construction in which the conflicting aspirations of unionists and nationalists were held in the most delicate balance.

“Now either there is a hard border between Northern Ireland and Britain or a hard border between the north and south of Ireland. And it is a shame and an outrage that peace in Northern Ireland is now treated as some disposable inconvenience to be bartered away in exchange for satisfying the obsession of the Brexiteers with wrenching our country out of Europe.”

Major, who has made several high-profile interventions against Brexit, said: “There’s a whole generation of people in Northern Ireland who have no memory of what life was like before the Good Friday agreement. No one under the age of 20 would have any memory of the violence and death that once engulfed their neighbourhoods. I hope and pray they never do.”

The People’s Vote march will set off from Park Lane at midday and reach Parliament Square at 2pm with a rally lasting 90 minutes for speakers to address the crowds.

It was scheduled for 19 October to coincide with the provisions of Labour MP Hilary Benn’s act. The Benn act legislated for Saturday to be the deadline by which a deal needed to be passed or the prime minister must request an extension of article 50 from the EU.

The rally may be one of the best attended of the three People’s Vote demonstrations so far, with a record 172 coaches bringing in campaigners from across the country. Previous marches are believed to have attracted between 500,000 and a million people.

Pauline Hardman, a food bank volunteer, is travelling to London for the first time from Ushaw Moor in County Durham – a former mining village.

She did not vote in the 2016 referendum but said she feared Brexit would damage former coal communities in the north.

She said: “People in mining villages like mine are getting more and more worried about the damage Brexit will cause. We can see it’s a million miles from what was promised back in 2016, we know this matters and we want to have our say.

“If you read the newspapers, you’d think that everyone in the north-east thinks exactly the same way, that we all want Brexit whatever the cost, but that’s not what I hear when I speak to my neighbours and friends.”

Another marcher will be Stephen Goodall, 97, who served in the far east during during the second world war and is travelling to the protest from his home in south Devon.

He said: “I’m 97 years old, my knees are knackered from playing rugby, and I’ll be in a wheelchair, but as long as I’m still alive and kicking, I’ll march to defend the democracy I’ve always fought to protect.”

The mayor of London, Sadiq Khan, will be supporting the event, as well as the shadow Brexit secretary, Keir Starmer, the shadow foreign secretary, Emily Thornberry, and the Labour MPs David Lammy, Benn, Jess Phillips and Chi Onwurah.

The Liberal Democrat leader, Jo Swinson, and her party’s MPs Ed Davey, Layla Moran, Luciana Berger and Sam Gyimah will be attending, as will the former deputy prime minister Michael Heseltine, the leader of the Independent Group for Change, Anna Soubry, the independent MP Antoinette Sandbach, Plaid Cymru’s Liz Saville Roberts, the SNP’s leader in Westminster, Ian Blackford, and the SNP MP Joanna Cherry.

Quick Guide How is Boris Johnson's Brexit deal different from Theresa May's? Show The new Brexit deal is essentially the old Brexit deal with a new chapter on the protocol on Ireland and Northern Ireland and a few key tweaks to the political declaration. Here is a link to the full text. The backstop is replaced The backstop has essentially been replaced by a full stop whereby Northern Ireland remains aligned to the EU from the end of the transition period for at least four years. A change can only happen if it is voted on by the Stormont assembly. Consent Stormont will have a key role in future Brexit arrangements. And if there is cross-community support to remain aligned to the EU rather than the UK the consent will hold for eight years. The arrangements in this deal will automatically kick in for a mandated four years if there is a breakdown in trade talks, so it remains a “backstop” but with a permanent tinge. That four-year period will start at the end of December 2020. Two months before the end of the four-year period, that is October 2024, Stormont will be asked to vote on whether to remain aligned to the EU in ways outlined by this deal or not. Checks on border, ports and airports Under the deal, the UK and the EU are “underlining their firm commitment to no customs and regulatory checks or controls and related physical infrastructure at the border between Ireland and Northern Ireland”. Future trade deals The EU and the UK will aim for a zero-tariff deal with unlimited quotas. The entire UK, including Northern Ireland, will be free to sign trade deals. The line in the political declaration that “the United Kingdom will consider aligning with union rules in relevant areas” in any future trade talks has been ditched. Customs Northern Ireland will remain legally in the UK customs territory but practically in the EU customs unions. There will therefore be no customs checks on the border but tariffs will be payable on certain commercial goods. No customs duties will be payable on “personal property” being transited from Great Britain to Northern Ireland. That protects online shopping and all items bought for personal rather than commercial use. Customs duties will be payable on goods imported from the UK for commercial use unless it can be demonstrated that the goods remain in Northern Ireland or are for personal use, as above. A system of rebates will allow importers to be reimbursed. West/east trade The commitment to frictionless trade between Northern Ireland and Great Britain is restated. VAT EU law on VAT will apply in Northern Ireland. Single electricity market The island of Ireland is considering a single market for electricity so homes in Northern Ireland can get their energy from a supplier in Northern Ireland or the republic. There were fears this could be disrupted by Brexit. Under the Johnson deal, the provisions of union law remain so nothing will change. Level playing field This guarantees that the UK will remain in line with EU conventions on climate, environment and workers rights in a future trade agreement. Lisa O'Carroll Brexit correspondent

The cost of a number of the coaches has been covered by supporters including the TV chef Delia Smith, who is paying for transport from Norwich and Bury St Edmunds. Patrick Stewart is paying for a coach from Huddersfield and the writer and director Armando Iannucci is meeting the cost of coaches from Oxford.

The Led By Donkeys Brexit message in Wiltshire. Photograph: Will Rose/Led By Donkeys/PA

The Brexit protest group Led By Donkeys unveiled its latest stunt to coincide with the crunch vote and march.

The words “Britain now wants to remain” was ploughed into a field in Wiltshire in 40-metre letters visible from passing aircraft.

The group released a video shot from a helicopter, with a voiceover by Lisa Dodd, who voted leave. She said: “I voted leave because of the NHS. I just can’t believe I fell for it. Manipulated wasn’t I, well and truly? I wonder how many more?”