Warnings that Theresa May’s Brexit deal could create a border between Northern Ireland and the rest of the UK are based on a myth, according to economic analysis.

The deal that MPs have rejected would keep trade between Britain and Northern Ireland flowing smoothly, with ports having to check on average just nine trucks a day, the study found.

The analysis by Northern Ireland business groups challenges a tenet of opposition to the deal negotiated by the prime minister.

The Democratic Unionist party (DUP) and Brexiter allies in the Conservative party voted against May’s proposed deal last month, saying it would undermine Northern Ireland’s position in the UK by erecting a de facto border in the Irish Sea. The EU has refused to renegotiate the deal’s legal text.

“All this aggravation and conflict is over nine loads a day,” said Aodhán Connolly, director of the Northern Ireland Retail Consortium (NIRC). “That is not the chaos they are predicting.”

The study found that of the 425,639 goods vehicles that entered Northern Ireland from Britain in 2017, 297,947, or about 70%, contained food.

If the UK leaves the EU under the terms of May’s deal, those vehicles would be assigned a 1% risk profile, the same afforded to shipments from New Zealand that are aligned to EU rules, according to the analysis.

That would equate to 2,979 goods vehicles needing to be inspected each year – an average of eight to nine trucks a day.

The figures were calculated by NIRC, Manufacturing NI and the Freight Transport Association based on data from the Northern Ireland Statistics and Research Agency. The groups plan to publish the figures next week.

Connolly said the findings debunked warnings that the backstop – a measure to avert a hard border between Northern Ireland and the Republic of Ireland – would create significant barriers between Northern Ireland and Britain.

“They’re making a big deal about Northern Ireland being hugely different from everybody else. The economic reality is this won’t make a lot of difference at all,” he said.

Most of the checks would be carried out at Cairnryan, a port in Scotland from which 15 ships sail to Northern Ireland each day. Another seven ships sail from Liverpool and Heysham.

About 60% of the trailers are shunted on and off the ships without drivers, so checks on nine trailers would affect perhaps just four drivers, according to the business groups’ analysis.

Stephen Kelly, the chief executive of Manufacturing NI, said checking nine loads a day was practicable and pragmatic. “It is workable and avoids huge costs for both business and government.”

Jeffrey Donaldson, a DUP MP, said the party respected Northern Ireland’s various trade associations but questioned the relevance of the analysis.

“There needs to be a dose of political reality here. Parliament overwhelmingly rejected the deal because of the backstop. Wide-ranging concerns about the backstop will not be overcome by producing figures on one element of those concerns, even if those figures are credible, and I don’t know if they are.

“This is about a lot more than how many trucks you stop at Belfast port, it’s about the constitutional and political integrity of the UK.”

The analysis is likely to widen the gulf between the DUP and business groups, which have lobbied in vain for the Brexit deal.

Seamus Leheney, a policy manager of the Freight Transport Association, said checks on freight must be minimised to avoid increased costs and delays. “The backstop appears to offer the most seamless solution in the event of it ever being required.”

The DUP leader, Arlene Foster, reiterated concern about regulatory checks between Northern Ireland and Britain after meeting the Irish taoiseach, Leo Varadkar, in Belfast last week.

She said: “The draft withdrawal agreement was unacceptable because it would cut Northern Ireland off from the rest of the United Kingdom and create a democratic deficit in Belfast.”

Brexit talks are expected to go the brink, with any last-minute offer by Brussels on the Irish backstop expected to be put to MPs just days before the UK is due to leave on 29 March.