An organisation representing the Polish diaspora in the UK has written to Theresa May urging her to ringfence the rights of EU citizens in the event of a “no deal” in Brexit negotiations.

The Federation of Poles in Great Britain has not voiced concerns before but told the prime minister in a letter that it felt “compelled to appeal” to her directly as fears grow that EU citizens would become illegal immigrants if the UK crashes out of the EU next March.

“The million-plus Poles in the UK, representing a good third of the EU citizens currently living here, have nervously studied the developing Brexit situation since the referendum with growing trepidation … They feel that they were overlooked and not involved in discussions which were to decide their future in the UK,” wrote Tadeusz Stenzel, the chair of the charity’s trustees.

He told her that the federation had noticed a “developing uneasiness, in particular among the parents” in relation to Brexit.

While there are significant numbers of Poles in the UK who fled the country after the second world war, the vast majority migrated following the accession of the country into the EU in 2004.

“Overwhelmingly these Polish citizens are UK taxpayers, making a substantial contribution to the British economy and to British society. This figure includes up to 200,000 Polish children, mostly born in the UK,” said the Federation which was founded in 1946 and is now an umbrella group with 53 member organisations.

The prime minister was told she could alleviate the uncertainty “at a stroke” by ringfencing the provisions for EU citizens in the draft Brexit withdrawal agreement she signed in Brussels in March.

The federation’s concerns are shared by many organisations lobbying for the rights of EU citizens, including businesses and the3million, a grassroots organisation set up after the referendum.

But the charity feels that some Polish migrants will be further disadvantaged because they will not be as well equipped as western European migrants because many have come to Britain for low-skilled work, may not speak fluent English and will not understand they have to register for a new “settled status” immigration category post Brexit.

“Hard-working Polish families in the UK have been deeply anxious about their fate ... but also they have been patient and trusting that the British government would keep to its promises that all EU citizens be allowed to stay,” said Wiktor Moszczynski, author of a guide to Poles in west London, Hello, I’m Your Polish Neighbour, and also a federation trustee.

He said Polish people were also concerned about discrimination post Brexit, feared that landlords and employers would claim they could no longer house them or hire them.