Campaigners for non-British or Irish EU citizens have made a formal complaint to the Electoral Commission amid fears many of them will be unable to vote in the UK in this Thursday’s elections for the European parliament.

Nicolas Hatton, the co-founder of the3million, has accused the regulator of providing insufficient guidance, leading to inconsistent advice from local authority electoral officers.

“The3million has multiple examples of local authorities mishandling the application process,” he said in a three-page letter.

Catherine West, the Labour MP for Hornsey and Wood Green, has raised concerns about the issue in a separate letter to Sir John Holmes, the chair of the Electoral Commission.

West fears that as many as 75% of non-British/Irish EU citizens living in the UK who registered to vote in the local elections three weeks ago could be turned away on Thursday because they have not filled in a second form necessary for them to vote in the European elections.

This is based on research in her own constituency, which found that just 25% of EU citizens who had registered for local elections and were not from the UK, Ireland, Malta or Cyprus had returned the second UC1/EC6 form.

Debra Jonckers, a Dutch national who had planned to vote in the UK, told the Guardian she returned the EC6 form by first-class post on 23 April. Alarm bells rang when her daughter’s polling card showed up on 8 May, the day after the voter register deadline, but cards for her and her Dutch husband did not.

Jonckers, an antenatal teacher, protested to the council in Maidenhead and was told her form had not been received. “The guy was very nice but he said they were a bit overwhelmed as they hadn’t expected to do this,” she said.

She said other EU citizens had told her of similar experiences. “I am so pissed off, it was my one chance to have a say on something that affects my life on so many levels. We have lived here for 32 years, chose to have our family here because we loved it and thought it was the best place in the world. The referendum was a slap in the face and we have no choices, no vote over our future,” she said.

Another voter said she complained to Camden council and was told the form had been sent out, but she never received it.

In her letter to the Electoral Commission chief, West said: “I fear that this process has helped create an artificial barrier to the enfranchisement of EU citizens, a concern shared by a number of my parliamentary colleagues.”

Hatton said the advice on the government website did not make it clear enough that the second form was needed.

“The3million believes that the Electoral Commission’s conduct has created unnecessary confusion and placed barriers in the way of exercising voting rights,” he said. He said the problem was compounded by some electoral registration officers (EROs) at local councils giving sometimes erroneous advice.

The Electoral Commission denied that it had been late in communicating with EU citizens. In its response to West, it said it wrote to EROs as early as 4 April, before Theresa May confirmed the elections would take place, to remind them that registration was more complicated for some EU citizens. “We advised EROs to identify such electors and send them a declaration form.”

It said any request for a change in the process would require a change in the law and was a matter for the government.