Theresa May’s promised “generous offer” on EU citizens’ rights at the start of the Brexit talks this week will fall short of protection of the status quo, meaning a difficult start to the negotiations, officials in Brussels fear.

The British government is yet to provide any documents relating to its proposal regarding the 3.1 million EU citizens living in the UK, but initial talks between the two sides have prompted concerns that Britain will make an underwhelming opening bid. Senior EU sources said the result would be a slow start to the already delayed negotiations.

In recent days EU officials have met Olly Robbins, the permanent secretary of the Department for Exiting the EU. The formal talks between the Brexit secretary, David Davis, and the EU’s chief negotiator, Michel Barnier, are due to start at 10am on Monday.

Ahead of the talks, the Department for Exiting the EU had let it be known that Davis was planning to make a “very generous offer” on the issue of EU citizens living in Britain.

The consequences of the UK offering less than permanent protection of all the rights currently enjoyed by EU nationals, however, could lead to reciprocal obstinacy, including the deprivation of the rights of elderly Britons in Spain to free healthcare, one official warned.

“We have a very strong suspicion, based on what we have heard, that the UK’s offer is about doing less than the current rights of these people,” the EU source said. “We will have to do a technical analysis when we see it, [asking] what are the shortcomings. Then we will have to reflect on what to do about that. There is no mandate to change our position.”

The source added: “I don’t see the Polish or Romanian governments saying, ‘they don’t want to give us what we would like, and we accept that.’ I don’t think that is going to be easy – I think that is going to take a certain amount of time.

“We have always had the suspicion that the UK would not be prepared to safeguard the rights of these people, but that they would like to do less than that. If the UK lowers the [offer] it will be difficult for us to discriminate against these people [Britons on the continent]. So I don’t know whether we would do that, but we might say, ‘OK, we will not provide health services free of charge’ – that might be a Spanish position, for example.”

The EU wants all rights currently enjoyed by EU nationals in the UK, and British nationals on the continent and in Ireland, to be protected in perpetuity, as long as they arrived in their adopted homes before Britain leaves the bloc at midnight Brussels time on 29 March 2019.

EU officials believe the British government will propose that the EU nationals who were in the country before the notification of article 50 on 29 March 2017 will have the same rights as UK nationals. That would mean future changes in laws affecting British citizens, including in relation to the ability of a spouse from abroad to join them in the UK, would affect EU citizens as well, leaving them potentially in a worse legal position than they are today.

Facebook Twitter Pinterest David Davis arrives in Downing Street. Photograph: Stefan Wermuth/Reuters

There are some doubts in Brussels, however, that Davis will even be able to make an offer, given the government’s loss of its Commons majority following the recent general election.

The Queen’s speech was due to be delivered on Monday but has been delayed to give the Tory minority government more time to strike a deal with the Democratic Unionist party.

The start of negotiations on Monday was dismissed by one EU diplomat as further “talks about talks”, rather than anything substantive. The source said: “I don’t know whether the UK will be in a position to table what they call a ‘generous offer’ on citizens. I think they are ready with it, but whether they are politically ready to do it I don’t know.

“So far they have been playing with the date of notification [as the cut-off for EU nationals to have their rights as citizens protected], saying they feared a rush to the UK. So far, that is not what we are seeing. We are seeing EU citizens fleeing.”

The EU believes that there is a distinct lack of clarity now on what the UK really is seeking out of the negotiations, three months after the letter triggering article 50 was delivered to the European council president, Donald Tusk.

The former prime minister of Denmark, Anders Fogh Rasmussen, speaking at a conference in Copenhagen, said Theresa May’s room for manoeuvre in the negotiations would be limited by her lack of a parliamentary majority. The UK would seek a softer break from the EU, Rasmussen added.

“The prime minister’s mandate for leaving the single market and the customs union can now be questioned. She has had to tear up most of her manifesto,” he said.