Lawyers representing the 94-year-old British expatriate living in Italy, Harry Shindler, and his co-claimant, Belgian resident Jacquelyn MacLennan, who launched the legal challenge on Wednesday against the rule barring long-term expatriates from voting in Britain’s European Union referendum in the High Court, have warned that those Britons living overseas who are not entitled to vote face being hit the hardest should Britons vote to leave the EU. For example, those working in EU members states could be forced to apply for work permits.

Lawyers from Leigh Day solicitors argue that the referendum franchise has already been extended beyond what is normal for elections and should also include expatriates. If successful, the law firm is demanding that the government "rush through amending legislation" before the June vote.

Richard Stein, the Leigh Day lawyer leading the case, says: "Our clients are being penalised for exercising their EU free movement rights. The EU Referendum Act 2015 is said to be based on legislation for UK parliamentary general elections. But it gives a vote in the EU referendum to members of the House of Lords, as well as to Irish and Commonwealth citizens who are resident in Gibraltar. None of these are allowed to vote in UK general elections.

"The people it arbitrarily excludes are those UK citizens who are among those most likely to be affected by the decision taken by voters in this referendum. Not to allow them to vote on the decision whether the UK remains part of the EU is unlawful and we have asked the court to deal with the issues urgently so that the Act can be amended before the June date, to include all UK citizens residing in the EU for however long."

There are currently an estimated 700,000 UK nationals living on the continent who are set to have no say on whether Britain stays in the EU. More than two million British citizens live on the continent and they fear losing health and residency benefits if voters decide to leave the European Union. A High Court judge will decide as quickly as possible whether to allow the judicial review to go ahead and, if granted, as the Bulletin reported yesterday, legal experts predict that legislation would have to be fast-tracked within a matter of days to register the extra voters on the electoral roll.

But the Conservative MP for Thanet North, Sir Roger Gale, who has played a large part in the campaign for all expatriates to be awarded the right to vote, warned that a ruling in favour of a judicial review could lead to the referendum being delayed. Leigh Day also stressed today that the need to register an extra 700,000 voters could threaten the proposed referendum timetable.

Sir Roger said that the European Commission has said that it is sympathetic to the challenge on the grounds that it infringes people’s human rights within the EU and the mere fact that the High Court agreed to five the review a first reading is a positive step in itself towards forcing the government to give all expatriates the vote ahead of the referendum.