The Dutch government has launched a damage-limitation campaign to try to counter what it fears is the disastrous international impact of the Islam-bashing populist Geert Wilders.

Wilders, whose success in June's general election catapulted him into the role of kingmaker in attempts to form a new coalition government, is to travel to New York to take part in protests on 11 September against the proposed Muslim community centre near Ground Zero.

Maxime Verhagen, the acting foreign minister and Christian Democrats' leader, has voiced fears that Wilders's speech in New York will tarnish Dutch reputations. He has also taken the unusual step of circulating confidential orders to Dutch diplomats around the world on how to answer questions about Wilders's influence in a new government and on the fallout for Muslims in the Netherlands.

With characteristic robustness, Wilders has told Verhagen to mind his own business. He clearly intends to grab attention with a tub-thumping exercise in Islamophobia in New York.

"Good feeling. Important speech. No one will stop me. No mosque at Ground Zero," he tweeted after booking a flight to New York. "Stop Islam, defend freedom" is his rallying cry.

The tensions over 9/11 and New York come as Wilders savours his growing clout at home. His Freedom party is running at 31% in the most recent opinion poll, ahead of all other contenders, and he has spent most of this week at a secret location with Verhagen and Mark Rutte, the liberals' leader, haggling over the terms for a new coalition government.

Wilders, whose party almost tripled its seats, from nine to 24, in the June election, is not joining the new cabinet. Instead, he will prop up a rightwing coalition of liberals and Christian Democrats in return for pledges of a tough new crackdown on immigration and other policy concessions. If the talks succeed, Wilders will be in the enviable position of wielding power while abjuring responsibility.

The negotiations have been going on for a fortnight and are supposed to be concluded next week. But they are said to be going badly.

A coalition backed by Wilders would command the slimmest of majorities – 76 seats in the 150-seat second chamber or lower house. The Trouw newspaper yesterday reported at least one dissident Christian Democrat MP would not support it, making it unviable.

Verhagen is in a difficult position. While negotiating with Wilders, he is also telling his diplomats how to undermine the rightwing maverick. Verhagen faces mounting resistance within his own party to collaborating, even if only tacitly, with Wilders.

Last week German Christian Democrats joined Dutch party dissidents in calling for a boycott of Wilders.

The latest opinion polls show Wilders soaring ahead of Rutte's liberal VVD party. Rutte, who is expected to be the new prime minister, supports an immigration crackdown and other anti-EU and hardline policies demanded by Wilders. But the two rightwingers are split over the main issue – austerity and budget cuts. Rutte is committed to slashing public spending by €18bn to halve the budget deficit from almost 7%. He is demanding health service, education, welfare and social security cuts.

Wilders, who is being prosecuted in Amsterdam on charges of inciting hatred and discrimination, is portraying himself as the protector of Dutch welfare, while calling for a tax on Islamic headscarves, a ban on the Qur'an, closure of Islamic schools, deportation of immigrants and proscribing mosque-building.

Verhagen has told his ambassadors how to cope with foreigners' questions such as "What will that mean for the treatment of Muslims?" if Wilders props up a new government.