Voters are being sent postal ballots with a guide that strongly suggests they should vote to for Britain to remain in the European Union, prompting furious complaints from anti-EU campaigners.

Voters in Bristol who have registered to vote by post have received a “how to vote by post” form as part of their voting packs which were sent out at the end of last week.

The step by step guide includes advice to “read the instructions carefully, then complete your ballot paper” above an image showing a pencil in a hand ticking a box to “remain a member of the European Union”.

    

There are now fears that the forms have been sent to postal voters in other council areas. Experts said there was risk that the forms could be challenged in court because they appear to guide the choice of voters.

More than six million people voted at last year’s general election alone which suggests that millions might vote in the EU referendum next month.

Henry Michallat, an 18-year-old student who received the form from Bristol City Council, said he was “appalled” that the guide appeared to guide people towards voting for the Remain side.

Mr Michallat – who stood as a Conservative candidate at May’s local elections – told The Daily Telegraph: “I am appalled by it. It should be neutral.

“When you see that bit of paper if you are not used to it or you are a first time voter, that might imply ‘of you should vote remain’ and put the cross in the box.

“When I first saw these instructions I was disgusted. The Electoral Commission should never have allowed this to be send. This is meant to be a democratic referendum and in my opinion these instruction contradict this.”

John Turner, chief executive of the Association of Electoral Administrators, said the forms could have gone out to tens of thousands if they were printed by a company which had contracts with other councils.

Mr Turner said: “Good practice would say that in any instructions to vote you do not put something which indicates you should vote in a particular way. Clearly this has not followed good practice.

“Whether it is unsafe or not only a court can determine. My guess is that is one of the big companies that do postal votes and they act for a large number of authorities.

“This is not going to be restricted to Bristol if what I think is the case.”

    

Bernard Jenkin MP, the Eurosceptic Tory chairman of the House of Commons Public Administration select committee which oversees the conduct of the referendum, said: “Any subliminal messaging by authorities purporting to be neutral is absolutely forbidden and it should be reported to the Electoral Commission.”

Arron Banks, one of the backers of the Leave.EU campaign, said: “To send out postal votes with instructions showing people how to vote and favouring IN campaign is the latest outrage.

“We have been made aware of this from a number of different concerned individuals over the last 24 hours who have asked us investigate.

“We will be asking our lawyers to contact the electoral commission for an explanation. We are already deeply concerned about the whole postal votes issue and will be calling for independent observers to monitor the process.”

The Electoral Commission said it would investigate if a complaint was made, saying the “how to vote” guide was organized by the council.

A spokesman said: “At a local counting area level they are allowed to add in instructions to help the voter – that is what has happened here. It could be something we have to follow up with Bristol.

“The counting officer can choose whether to set out more about the completion of the postal ballot in another set of instructions – whether they add in pictures or not is up to them.”

A spokesman for Bristol City Council confirmed it produced the “how to” form, saying: “I can confirm this form was produced on behalf of Bristol City Council.

“This form is designed to explain the logistics of voting by post and not to suggest how someone should vote.

“The placement of a pen graphic is incidental. No cross (X) is shown, so it could not in our view be construed as indicating how to vote.”