One in five farmers would fail to break even if European Union subsidies were halted, the prime minister has claimed ahead of a visit to the region today.

David Cameron said European Union funding for agriculture could not be guaranteed if Britain voted to leave.

But his claims come as 13 of his own ministers, who are all campaigning for Britain to leave the EU, have pledged to match the current level of European Union funding for farmers, as well as for scientists, universities and regional organisations.

The politicians, including former London mayor Boris Johnson and justice secretary Michael Gove, have co-signed a letter claiming the funding for organisations, which currently receive money from the European Union, is safe if Britain voted to leave.

'There is more than enough money to ensure that those who now get funding from the EU – including universities, scientists, family farmers, regional funds, cultural organisations and others – will continue to do so while also ensuring that we save money that can be spent on our priorities,' they said in the open letter. The East of England received £292m in CAP grants in 2014.

But Mr Cameron countered: 'As long as I am prime minister, I would try to ensure that an agricultural support system would be maintained. But all the experts argue that leaving the EU wouldn't save us money, it would cost us money because a smaller economy means less tax revenue.

'Savings would have to be made. And there would be nothing to stop a future government from removing this support altogether.'

But the letter, also signed by Essex MP Priti Patel, said: 'Many areas have seen recent falls in EU funding. The value of agricultural support is in decline and EU structural funds have been cut back significantly. The real danger to current recipients of funding from the EU institutions is that if we vote to remain the EU will further reduce their funding.

'After protecting those now in receipt of EU funding, we will still have billions more to spend on our priorities. We propose that at least £5.5bn of that be spent on the NHS by 2020, giving it a much-needed £100m per week cash transfusion, and to use £1.7bn to abolish VAT on household energy bills.'