US travel warning: Full list of countries advising 'caution' when visiting  America - including the UK, France, Germany and Denmark - in backlash at Trump's 'two gender' decree

European countries are warning citizens with a certain type of passport to proceed with 'caution' when travelling to America in a backlash at Donald Trump's 'two gender' decree.

A number of U.S. ally nations are urging their trans and non-binary residents, who have 'X' listed as their gender on their passport, to seek guidance from American embassies before entering the country.

It comes after Trump banned 'X' gendered U.S. passports and signed an executive order declaring there are only two genders in January.

The Republican's declaration has confused countries that allow the passports and sparked fears that some 'X' gendered travelers may be refused entry to the U.S.

Many nations have been forced to update their travel advisory pages in response to the stricter entry rules.

Germany, Finland, the UK, and Canada have all revised their official guidance for citizens visiting the U.S. - particularly for those who may be affected by the policy change.

These countries highlighted risks for transgender and nonbinary travelers, advising caution or in some cases, direct consultation with U.S. embassies before departure.

The UK's Foreign, Commonwealth and Development Office warned British citizens to fully comply with all entry and visa requirements and stressed that any violation -intentional or not - could result in arrest.

Meanwhile the French foreign ministry tells nationals to declare their gender assigned at birth when applying for a U.S. visa or through the Electronic System for Travel Authorization (ESTA), to comply with Trump's order.

Denmark has gone one step further than France, explicitly warning that transgender people may be barred from entering the United States.

The Danish foreign ministry warns that travelers whose passport gender does not match their assigned sex at birth could be turned away at the border.

Finland also updated its travel advice.

'If the applicant's current sex on their passport is different from their sex confirmed at birth, the US authorities may refuse entry,' a federal Finnish travel site warns.

President Joe Biden 's State Department was the first to allow applicants to label their gender as 'X', but that was rolled back quickly by Secretary of State Marco Rubio after Trump's initial order.

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'The policy of the United States is that an individual's sex is not changeable,' the State Department said in a statement at the time.

Trump's gender executive order also advised the Department of Homeland Security to implement changes for government issued IDs.

The options are either male or female.

Even countries with long-standing military and economic ties to the U.S., like Germany, have been issuing warnings.

The German Federal Foreign Office has notified that residents with the 'X' gender on their travel documents may face issues when coming to America.

Germans traveling to the U.S. must state 'either 'male' or 'female' on ESTA or visa applications,' the travel guidance states.

It also advises those with 'X' passports to check their documents with U.S. officials before entering the country.

In U.S. customs enforcement, border officials ultimately have the final say over which travelers get to enter the country — which is noted on the German travel site.