Ireland will have 400 new customs officers in place by the end of March as it rushes to put no-deal measures in place to keep trade moving with Great Britain, according to an unpublished document from the Irish tax authority.

Niall Cody, the chairman of the Revenue Commissioners, will reveal the scale of no-deal contingency operations for the first time on Thursday, warning of the adverse impact for 70,000 Irish small to medium-sized businesses that trade with the UK.

“We have accelerated and expanded our recruitment and training schedules to meet the end of March deadline [for Brexit],” he will tell a parliamentary committee.

But the Republic says it will not be putting any customs posts on the border with Northern Ireland. “The government has made clear that the overriding objective is to avoid a hard border on the island of Ireland. Revenue is not planning for customs post,” Cody will say.

PMQs verdict: Corbyn leaves May sounding defensive on Brexit Read more

“The government has indicated that in the event of no deal, it will engage in intensive discussions with the EU commission and our EU partners will provide whatever technical expertise and assistance may be required during this process.”

His comments come after the EU’s chief Brexit negotiator, Michel Barnier, said in the event of no deal the EU “will have to find an operational way of carrying out checks and controls without putting back in place a border”.

It seemed to backpedal on earlier claims that no deal would lead to a hard border, raising questions about the lack of a clear definition of that term, and the absence of no-deal planning papers in the public domain from the UK, Ireland or the EU in relation to the most contentious issue in the Brexit deal.

During 2017, 84,000 Irish businesses traded with the UK and about half of those, 45,000, were regular traders. Only 17,000 businesses in Ireland currently trade with third countries, requiring 1.6m customs import and export declarations to be processed.

“Post Brexit, import and export declarations could increase to as many as 20m per annum,” Cody will say on Thursday. This is because the UK will be classified as a third country from 30 March in the event of a no deal.

The Labour TD and former deputy prime minister of Ireland Joan Burton said this would spell chaos not just for Ireland but for the UK as well. “I think that finally we are being honest about what happens if the UK leaves the customs union and the free movement of goods ends until alternative arrangements are made,” she said.

“If Ireland has to deal with 20m import and export declarations and we are a small country, just think of the number of import and export declarations the UK will have to cope with with France, the Netherlands, Germany, all 27 countries.”

Burton said infrastructure on the Northern Ireland border was in “nobody’s interest including the unionist community”.