The British people must be told the brutal truth about the damaging consequences of leaving the European Union, and not be duped into believing that they can "keep all the goodies and forget about the costs", the Polish minister in charge of EU policy says.

In an outspoken interview with the Observer, Rafał Trzaskowski, Poland's secretary of state for European affairs, who met David Cameron 10 days ago in Warsaw, says that if political leaders do not tell the people of the UK the facts in advance of the planned in/out referendum, many will vote to leave, unaware of the devastating consequences.

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The minister, who reiterates his country's refusal to accept Cameron's central demand – that social benefits should be denied to all EU migrants for at least four years after arriving in the UK – says Britain would no longer be an important player, in Europe or the world, if it left the EU. He warns that the ability of British people to travel as freely as they do now, and to work and buy homes in other EU countries, would also be lost, and that UK businesses would suddenly face new problems, as the country would no longer be able to influence the rules of the internal market.

Trzaskowski, reflecting growing fears in the EU that the UK government is setting itself unrealistic targets for reform which British people are being led to believe are achievable, says all European leaders want Britain to stay in the EU, but not if it means undermining EU principles, such as the free movement of labour. "No one in Europe says, 'We don't care, take it or leave it.' No, we say, 'Guys, OK, some of your fears are justified, some of your sensibilities should be taken into account, and some of your idiosyncrasies, even though for us they might seem strange. We should talk about it.' We do not say ship out. But there are certain red lines. We cannot start questioning the cornerstones of integration, because it will have far-reaching consequences."

Asked whether he fears the British people might vote to leave, he says: "It really depends on how the whole thing is depicted by British political parties to the British people – what kind of alternative is painted. If you say you can leave and still be part of the internal market, keep your [second] houses, that you will still be free to travel, that there will be no customs duties, and so on – but that you will not have to accept free movement of workers, and you will not pay into the EU budget, of course people will vote [to leave] … but this is simply not true."

Trzaskowski adds: "If Great Britain leaves, it will minimise its role. I am not trying to be patronising. I think we have to be frank with the British people when we talk about their future in the European Union. Many people in Europe want to be accommodating … but if the demands are too extreme, they are not going to be met.

"You cannot keep all the goodies and forget about the costs. Britain will still have to pay into the EU budget, just as the Swiss and Norwegians do. It would have no influence over the decisions yet it would have to subscribe to all the rules. London would not be that sexy a place for capital movement because it would have much looser links to Europe."

German chancellor Angela Merkel suggested last week that her country would do all in its power to try to meet British demands during a wide-ranging renegotiation of its membership ahead of the referendum. But it is understood that Berlin also believes that the key British demand – to deny migrants who are in work the same social benefits as UK residents – would amount to a fundamental breach of EU law.

Cameron is now urging fellow European leaders to reach an outline deal on the UK's demands, which also include an opt-out from the EU commitment to "ever-closer union", by the end of this year, in time for the referendum to be held next year. Should he fail to secure reform on his terms, more than 50 Tory MPs are poised to lead the campaign for the UK to quit the EU. Former cabinet ministers Owen Paterson and John Redwood are among backers of the newly formed Conservatives for Britain (CfB) that will initially support the prime minister's bid to renegotiate terms of membership for the UK, according to the Sunday Telegraph.

Poland, whose economy has strengthened since it joined the EU in 2004, now sees itself as a central player in the union and is determined to defend the rights of the 700,000 Polish citizens in the UK, the vast majority of whom are in work.

Trzaskowski says that just because Cameron feels strengthened by his general election victory, Poland will not roll over. "We cannot say simply that we sit around the table and you say, 'This is my manifesto and