Imposing curbs on in-work benefits for EU migrants would be discriminatory and would fail to deter migrants from coming to Britain, the Polish ambassador to the UK has said.

As the UK prime minister, David Cameron, prepares to deliver a long-awaited speech on EU migration in which he is expected to propose limiting access to tax credits, the Polish ambassador, Witold Sobków, indicated that Britain would fail to secure agreement among fellow EU leaders for the changes.

But the Eurosceptic Tory MP Bernard Jenkin said Britain must press for reforms because the UK had become a “honeypot” for migrants. He was speaking as Owen Paterson, the former environment secretary, calls on Cameron to strengthen Britain’s hand in its EU renegotiations by triggering exit negotiations at the start of the process.

In a speech in London, Paterson will say that triggering article 50 of the Lisbon treaty, the exit clause, is the only “legally binding” way to put pressure on the EU to embark on serious negotiations that would give Britain the chance to leave the trade bloc’s formal political structures.

Poland, a historical British ally, said Cameron would struggle to win support during the renegotiations for his more modest proposals to restrict access to in-work benefits such as tax credits and housing benefit.

Sobków cited research in a report by the Open Europe thinktank, which has called for in-work benefits for migrants to be curbed for two to three years. It showed that out of 3.26 million working families claiming in-work tax credits only 252,000 come from EU member states outside the UK.

The ambassador told BBC Radio 4’s Today programme: “People do not come here for benefits. They come here to work. The vast majority of migrants come to the UK to work. This is what the report says. They have no idea about benefits.”

Asked whether curbs on in-work benefits on EU migrants would be discriminatory, Sobków added: “Of course. We pay the same taxes which pay for tax credits doing the same job. Imagine you have three people working for the BBC, one from Spain, one from Poland and one from the UK. They live here and they pay taxes here. Why should you discriminate against the Spanish and the Polish worker? Give me one reason.”

But Jenkin said curbs needed to be imposed on EU migration because Britain was attracting large numbers of workers because of the failings in the eurozone. He told Today: “The huge unemployment in southern Europe now, particularly among the young who are most mobile, because of the stagnation of the whole eurozone [means] we have become a honeypot nation in the EU. [That] is why we have done well controlling immigration from non-EU countries but that has been offset by quite a dramatic increase in immigration from EU countries.”

The home secretary, Theresa May, admitted on Sunday that the UK’s inability to control EU immigration meant it would fail to deliver the prime minister’s “no ifs, no buts” pledge to reduce net migration to the tens of thousands by the general election next year.

Speaking on the Andrew Marr Show on BBC1, she said: “It is, of course, unlikely that we are going to reach the tens of thousands by the end of the parliament. Why is that? It is because we have seen increasing numbers of people coming from across Europe, partly because our economy is doing better than other economies across Europe. We have been doing what we can in relation to EU migration, but there is more to be done.”

The home secretary confirmed the prime minister would demand restrictions to the free movement of people when he renegotiates Britain’s EU membership terms.

But Paterson will urge Cameron to go much further and to negotiate a relationship with the EU that falls short of full membership. He told the Times: “The eurozone has already embarked upon a path that we can never follow. We are simply recognising that reality. We must either be fully committed to ‘Le Project’ or we must build an entirely new relationship. The British people must be allowed to make that decision. Article 50 is the only way of making that happen.”

The former environment secretary added: “We were told that it wasn’t a political project – only a common market – but Macmillan, Wilson and Heath knew that it was. They misled the people. People voted to join because politicians told them that it was the answer to Britain’s economic troubles. But France and Germany were growing strongly before the formation of the then European Coal and Steel Community.

“We now have only 8% of the votes on the council of ministers. If we were an independent nation we would be able to have our own seat on the world trading bodies – like Canada does, like New Zealand, like Australia.”