Anti-Brexit campaigners have seized on a bleak Whitehall assessment of the economic impact of leaving the European Union, published following a battle over government secrecy.

MPs voted in January for the document to be released in full, but its publication was resisted by the Brexit secretary, David Davis.

The cross-party Brexit select committee issued the document, which was prepared to aid thinking inside government about possible scenarios, on Thursday.

As leaks had suggested, government number crunchers found that economic growth would suffer under any of the existing models for a future relationship with the EU – a Norway-type European Economic Area (EEA) model; a free trade agreement; or trading on the basis of World Trade Organisation (WTO) rules alone.

Under the worst scenario – a WTO-type arrangement – GDP could decline by a cumulative 7.7% over 15 years, the analysis found; while under an EEA deal it would be 1.6%.

The committee’s chair, the Labour MP Hilary Benn, said: “The results of this analysis, undertaken by the government with the aim of quantifying the potential impact of leaving the EU on the British economy, are already largely in the public domain in one form or another.

“Allowing this information to be considered in its full context, rather than selectively quoted, will help properly to inform public debate about how the figures were arrived at and what the economic effects of Brexit might be.

“The analysis suggests there will be an adverse effect on the economy of the UK and all its regions, and that the degree of impact will depend on the outcome achieved in the negotiations.”

However, pro-Brexit MPs questioned the credibility of the forecasts. Jacob Rees-Mogg, who chairs the backbench European Research Group, said the research had been “so widely leaked and ridiculed for its approach that it is of little consequence”.

Whitehall officials warn in the document that forecasts are highly uncertain, given the “unprecedented” nature of the deal the government hopes to strike, but say economic forecasts will be necessary to inform the negotiations.

Quick Guide What are Brexit options now? Four scenarios Show Staying in the single market and customs union The UK could sign up to all the EU’s rules and regulations, staying in the single market – which provides free movement of goods, services and people – and the customs union, in which EU members agree tariffs on external states. Freedom of movement would continue and the UK would keep paying into the Brussels pot. We would continue to have unfettered access to EU trade, but the pledge to “take back control” of laws, borders and money would not have been fulfilled. This is an unlikely outcome and one that may be possible only by reversing the Brexit decision, after a second referendum or election. The Norway model Britain could follow Norway, which is in the single market, is subject to freedom of movement rules and pays a fee to Brussels – but is outside the customs union. That combination would tie Britain to EU regulations but allow it to sign trade deals of its own. A “Norway-minus” deal is more likely. That would see the UK leave the single market and customs union and end free movement of people. But Britain would align its rules and regulations with Brussels, hoping this would allow a greater degree of market access. The UK would still be subject to EU rules. The Canada deal A comprehensive trade deal like the one handed to Canada would help British traders, as it would lower or eliminate tariffs. But there would be little on offer for the UK services industry. It is a bad outcome for financial services. Such a deal would leave Britain free to diverge from EU rules and regulations but that in turn would lead to border checks and the rise of other “non-tariff barriers” to trade. It would leave Britain free to forge new trade deals with other nations. Many in Brussels see this as a likely outcome, based on Theresa May’s direction so far. No deal Britain leaves with no trade deal, meaning that all trade is governed by World Trade Organization rules. Tariffs would be high, queues at the border long and the Irish border issue severe. In the short term, British aircraft might be unable to fly to some European destinations. The UK would quickly need to establish bilateral agreements to deal with the consequences, but the country would be free to take whatever future direction it wishes. It may need to deregulate to attract international business – a very different future and a lot of disruption.

The international trade secretary, Liam Fox, upped the ante on Thursday as he told business groups at the British Chambers of Commerce annual conference that the EU27 was acting like a “gang”.

“The idea of punishing Britain is not the language of a club, it’s the language of a gang,” Fox said. “We need to begin this argument by putting politics aside and do what is in the economic interests of the people we represent.”

The Democratic Unionist party’s Westminster leader, Arlene Foster, also toughened her language, attacking the EU’s hardening stance on trade and the Irish border at the same event.

In a speech criticising the European commission’s “bad faith” on the border issue, Foster hit out at the view of Brussels leaders that failure to accept proposals for keeping the border open would encourage a return of terrorism – which, she said, was tantamount to a threat.

She told the conference: “I do object, in the strongest terms, to people who have limited experience of the Troubles in Northern Ireland throwing threats of violence around as some kind of bargaining chip in the negotiating process. To do so is an insult to the people of Northern Ireland who have worked so hard to bring peace to our country.

“I can remember the evening when the IRA shot my father [and] in bloodstained state he crawled into our kitchen. I remember the day the IRA bombed my school bus severely injuring the girl sitting beside me. When I talk about the border in a Brexit scenario, I don’t speak about some far away land; I speak about home.”

The Whitehall Brexit analysis points to potential border checks as one of the “non-tariff barriers” that are the “most material consideration” in assessing the potential costs of post-Brexit trade.

In the retail sector, for example, officials calculate that these non-tariff barriers could be equivalent to the EU slapping tariffs of up to 20% on British exports, the document suggests – with chemicals facing tariffs well above 10%.

This analysis helps to explain why Theresa May said in her speech last week that Britain was keen to remain under the auspices of some key EU regulatory bodies.

The document points out that officials have built a new economic model to assess the long-term impacts of changes to trade relationships, underlining the fact that the analysis does not simply rehash the Treasury’s “project fear” forecasts from the referendum campaign.

Stephen Doughty, a Labour member of the committee, said: “Now the public can see for themselves the hugely damaging impact of the reckless course the government is pursuing.”

As already reported, the analysis suggests the long-term gains from striking new trade deals with non-EU countries – a central plank of the pro-Brexit case – could be just 0.2%-0.7% of GDP. That would probably be dwarfed by the losses from a looser relationship with the EU, officials believe.

However, Aarti Shanker, of the thinktank Open Europe, played down the scale of the expected downturn, saying the UK government could “use other domestic policy levers,” to help offset the impact.