Farming leaders have accused the Chancellor of refusing to engage with them over controversial changes to inheritance tax, as thousands of farmers marched on Westminster.

The National Farmers’ Union (NFU) is holding a mass lobby of MPs with 1,800 of its members – three times as many people as originally planned – to urge backbenchers to stand up to the Government’s plans to impose inheritance tax on farms worth more than £1 million.

Many more joined a separate rally in Whitehall as they protest against last month’s Budget, which also sped up the phase-out of EU-era subsidies as funding is switched to nature-friendly farming schemes.

The Met Police estimated more than 10,000 people had joined the rally in central London shortly before midday – with “more arriving”.

Farmers have reacted with anger and dismay to the inheritance tax changes for farming businesses, which limit the existing 100% relief for farms to only the first £1 million of combined agricultural and business property.

Some warn they will have to sell off land to meet the costs and are threatening to strike over the pressures they say they are being put under by Government policy, while there are warnings over people’s mental health.

NFU president Tom Bradshaw gave a speech to members taking part in the mass lobby ahead of their meetings with MPs, at Church House, Westminster, in which he became emotional at moments as he highlighted the cost of the policy on farmers, as well as food security.

He said: “We know the horrendous pressure it is putting on the older generation of farmers who have given everything to providing food for this country.

The human impact of this policy is simply not acceptable, it’s wrong

“We know that any tax revenue will be taken from our children and raised from those that die in tragic circumstances, all within the next seven years.”

To sustained applause, he said: “The human impact of this policy is simply not acceptable, it’s wrong.”

He warned the policy was “kicking the legs out from under British food security”.

And he said: “Our request is very simple, this is a policy that will rip the heart out of Britain’s family farms, launched on bad data with no consultation, and it must be halted and considered properly.”

Speaking to journalists after his speech, Mr Bradshaw was quizzed about the apparent impasse between ministers who are defending the policy and farmers, who say it must be rethought.

“We’ve offered to meet with the Treasury to give them solutions to this, unfortunately, so far, Rachel Reeves has refused to engage,” he said.

He refused to spell out publicly how he thought it could be resolved, but said: “I’ve given a heartfelt plea to the Chancellor to sit down with me so that we can sort this policy out.

“The human impact of this policy is one I don’t believe they intended but the longer they leave this hanging, the more I start to think it’s vindictive, rather than miscalculated.”

Mr Bradshaw said the latest analysis by the NFU suggested 75% of commercial farms, over 50 acres, would be affected by the changes.

But speaking to the PA news agency, Environment Secretary Steve Reed insisted only a few hundred farms would be impacted, rather than the tens of thousands claimed by critics of the inheritance tax change.

And he denied that Labour, which has many more MPs in rural areas since the general election, did not understand the countryside.

He told PA: “This Labour Government has just allocated £5 billion to support sustainable food production in the UK.

“That’s the biggest budget of that kind in our country’s history and it shows that we’re backing farmers,” he said, and outlined other measures to support farmers on issues such as flooding and trade.

The changes to inheritance tax will affect only around 500 farms. The vast majority of farmers will pay nothing more

“All of that shows farmers that this is a government on their side and the changes to inheritance tax will affect only around 500 farms. The vast majority of farmers will pay nothing more.”

Outside on the streets of Whitehall, celebrities including Jeremy Clarkson – who has previously said avoiding inheritance tax was “the critical thing” in his decision to purchase farmland but told PA on Tuesday it was because he wanted to shoot – joined farmers for the protest.

Speaking at a photocall of Tory MPs and farmers as the protest kicked off in London, shadow chancellor Mel Stride told PA: “We’re doing this to show solidarity with our farmers.

“We believe that this Government doesn’t understand the countryside or farming, broke its promise when it said that it would not be imposing inheritance tax on farms, it has now done that.”

His colleague, shadow environment secretary Victoria Atkins, said she had given ministers at a meeting on Monday night an example of one elderly farmer considering whether “it would be better for him to pass away” before the inheritance tax changes come through and “they had no answer”.

And a fourth-generation family farmer said there is a possibility he and other farmers will strike if changes to agricultural property relief are not reversed.

Richard Wainwright, 58, from Halifax, West Yorkshire, whose grandfather began farming with a few cows and delivered milk to the surrounding area, told PA: “We are talking about possibly striking.

“I hope it doesn’t come to that because that’s seriously going to impact the food chain,” he said, as he warned the family would possibly have to sell 20% of the farmland to cover the tax bill of around £600,000.

As he joined the protest, Mr Clarkson said: “I’m here to support the farmers, it’s that simple, because they need all the help they can get really, even from me.”

He said he opposed the tax changes, adding: “If she’d have wanted to take out the likes of James Dyson and investment bankers and so on, she would have used a sniper’s rifle, but she’s used a blunderbuss and she’s hit all this lot.

“It was – as I understand it – it was a very rushed last-minute decision and I think we all make mistakes in life, and I think it’s time for them to say ‘you know what, we’ve cocked this one up a bit’ and back down,” he said.

A Government spokesperson said: “Ministers from both HMT (Treasury) and Defra have regularly engaged with the NFU, including the Secretary of State, Farming Minister and Exchequer Secretary.”