David Cameron would be able to negotiate major reforms to Britain's EU membership terms even if the prime minister was defeated over the appointment of Jean-Claude Juncker as the next European commission president, Poland has said.

As an unstoppable momentum appeared to be building up behind Juncker, as centre-left EU leaders endorsed him ahead of an EU summit later this week, the Polish foreign minister said Britain could win support for "sensible" reforms.

Radek Sikorski, an ally of the Tories who was a member of the Oxford Bullingdon Club at the same time as Boris Johnson, said Juncker should be appointed because he was the candidate of the main EPP grouping which came first in the European elections. Sikorski said it was "unfortunate" that Cameron had been unable to influence the appointment of Juncker after he withdrew the Tories from the EPP, which is the main centre-right group in the European parliament.

Sikorski, whose Civic Platform party is a member of the EPP, told the Andrew Marr Show on BBC1: "If the Tories were part of the EPP he could have made that argument [against Juncker] at the Dublin summit when the EPP chose its candidate. He may well have prevailed. But the EPP made its choice, won the election and here it is. The rules of the democracy are that the largest party gets the top job."

But Sikorski said a defeat over Juncker's appointment would not harm Cameron's hope of renegotiating Britain's EU membership terms. The Polish foreign minister said: "I think Britain can gain a lot of support and allies on the continent for sensible British proposals to allow nation states the decision-making power. We call it, in the Brussels jargon, the subsidiarity principle – to leave what is possible at the level of the member state and then do together those things where we all gain by working together, say on energy and defence."

Sikorski said the Tories would win support to tighten benefit rules because such rules already existed in continental countries. But he described the idea of welfare tourism as a "figment of some politician's imagination" because Poles in Britain worked and sent back earnings that have been taxed.