Consensus is growing among pro- and anti-EU campaigners that the referendum on Britain’s membership of the bloc will not be held until September, amid signs that a deal on David Cameron’s reform proposals will be delayed until March.

As the prime minister faced calls from Viktor Orbán, his Hungarian counterpart, to alter his proposal to ban EU citizens from claiming in-work benefits for four years, sources on both sides played down the prospect of a deal by No 10’s preferred date of February.

The two camps agree that more work will need to be done to address concerns in eastern and central Europe that the benefits proposal is discriminatory.

A failure to reach a deal at an EU summit next month would push the negotiations to the annual spring European council in mid-March, making September the earliest date that a referendum could be held.

At least 16 weeks have to elapse between the conclusion of the negotiations and the referendum to allow for the passage, over a period of four to six weeks, of secondary legislation setting the rules of the campaign.

The campaign will last at least 10 weeks. That would make July the earliest date for the referendum. But July has been ruled out because the Scottish school holidays are under way then, leading the campaigners to work on the basis that the referendum is likely to be held on 15 September.

A referendum in September could be politically difficult for Cameron if he has secured a deal with fellow EU leaders and is campaigning for a yes vote. A fresh summer migration crisis along the lines of last year’s, when refugees from the Syrian civil war sought to make perilous journeys across the Mediterranean, could be at its height by then, handing useful ammunition to opponents of the EU.

But ministers know that September is the last realistic date because a referendum would be difficult in 2017. French and German elections are due to take place in the spring and late summer respectively next year.

Campaigners on both sides are working on the basis that a deal is most likely to be will agreed at March’s EU summit. One pro-EU campaigner said: “February is not that long away. You have got to get a lot of detail worked out. February is looking a little ambitious.”

An anti-EU campaign source said: “There is a lot to do to get it all done by February. No 10 is just saying it wants a deal in February to keep up the momentum.”

The challenge facing the prime minister was highlighted when Orbán said it was important to avoid portraying EU workers in the UK as parasites.

Orbán made the remarks at an otherwise upbeat press conference with Cameron in Budapest, in which he said he agreed with three out of the four “baskets” of EU reform proposals tabled by Britain.

His only disagreement is over the plan to ban EU workers from claiming in-work benefits for four years. But Orbán indicated that Hungary and the other three Visegrad group countries – Poland, the Czech Republic and Slovakia – were confident they would reach an agreement with Britain on the welfare reforms.

Orbán said: “For us it is very important that we are not considered as migrants. Words matter here … So we would like to make it quite clear that we are not migrants into the UK. We are citizens of a state that belongs to the EU who can take jobs anywhere freely within the EU … We do not want to be parasites.”

He balanced his warning by saying the Visegrad countries were determined to reach a deal with Britain. This could involve imposing a residency test which would mean that UK citizens would also be banned from claiming tax credits for the first four years of their working life.

Orbán also spoke of the need to consider the “very special British” system. This suggests that Cameron has had some success in explaining the political challenge posed by the UK’s universal benefits system. Benefits are available immediately to UK and EU citizens, unlike in many other EU countries that have contributory systems.

Downing Street was delighted by Orbán’s upbeat tone. Cameron said he was willing to consider other proposals on welfare reform. But they would have to deal with the “artificial draw” of the UK’s non contributory welfare system.

The prime minister said: “My proposal remains on the table but I am happy to look at alternatives if they provide real solutions.”

Downing Street is still officially committed to reaching a deal at the February EU summit, keeping open the option of a June referendum. But its handling of the parliamentary rules for a referendum suggests that it has given up on a referendum before September.

Ministers have not yet begun the process of introducing the secondary legislation to allow the referendum to take place. The primary legislation reached the statue book in December.

The electoral commission says six months need to elapse between the introduction of the secondary legislation and the referendum. The failure to introduce the secondary legislation so far suggests that the government has abandoned any hope of holding the referendum in June.

