Angela Merkel will use a high-profile visit to London on Thursday, in which she will address a joint session of parliament and have tea with the Queen at Buckingham Palace, to plead with the British people to remain members of the European Union.

Amid genuine fears in Berlin that Britain may be on a trajectory towards exit, the German chancellor is expected to say that Britain benefits from its membership of the EU. But Merkel is also expected to say that the EU benefits from Britain's open approach to trade and markets.

Her visit, which has many of the trappings of a state visit rarely offered to a head of government, contrasts with the low key reception for the French president, François Hollande, at the Anglo-French summit last month at RAF Brize Norton. Hollande faced embarrassing questions about his private life at a press conference with David Cameron and then sat through an awkward and short lunch with the prime minister at a nearby upmarket gastropub.

Merkel will address a joint session of parliament at midday before having lunch with the prime minister in Downing Street. They will hold a joint press conference before Merkel holds separate meetings with Nick Clegg and Ed Miliband.

Cameron has invested a great deal of capital in Merkel in the belief that she will be the pivotal figure if he wins the 2015 general election and seeks to renegotiate Britain's EU membership terms. He tried to help Merkel in her re-election campaign last year by offering to give her the full red carpet treatment in London before the election. But Merkel said she would rather wait until this year.

The German chancellor advised Cameron last year to cast his landmark speech on the EU, in which he pledged to hold an in/out referendum on Britain's membership of the EU by the end of 2017, as a campaign to reform the EU as a whole. Merkel has told the prime minister that – if he acts in that spirit – she would be prepared to offer help if the Lisbon treaty were revised to underpin new governance arrangements. A senior Berlin official has said that assurances could be introduced during modest treaty revisions to ensure that the interests of Britain and other non-euro members are protected in the European single market.

But one senior coalition figure said any concessions offered by the Germans would amount to "chickenfeed" that would not satisfy Tory eurosceptics. "David Cameron will get some concessions but it will lack powder and shock. There is an interesting parallel with Harold Wilson and Jim Callaghan before the 1975 referendum, who trotted round the capitals of Europe getting a packet of matches rather than a cigar and hailed this as a great triumph."

"David Cameron is caught. He goes to Brussels and gets chickenfeed – that won't satisfy the eurosceptics. He goes there and argues for something more than chickenfeed and his bluff is called."

Willy Brandt was the last German chancellor to address both houses of the British parliament, delivering a speech in 1970 hailed in a Guardian leader as "an unqualified success". Summing up Brandt's visit to Britain on 2 March 1970, this newspaper wrote: "It is doubtful if relations between Britain and Germany have ever been better – not on a basis of overflowing sentiment, but in terms of quiet trust, patience and understanding."

It is unlikely the British media will be championing Merkel's visit in similarly euphoric tones by the end of the week. For a start, Britain was then trying to negotiate its way into the EEC. When Merkel speaks in Westminster on Thursday lunchtime, many of those on the benches in front of her would prefer Britain negotiate its way out.

Those wanting Britain to stay in, such as the prime minister, will be hoping that Merkel will throw them a lifeline, a trophy those wanting to stay inside Europe can produce as evidence for having successfully renegotiated Britain's relationship with Brussels.

But at the top in Germany there is a growing concern this week that British expectations have grown far too high for the chancellor to meet. The room Merkel had to voice her support on Thursday was limited, said Almut Möller of the German Council on Foreign Relations thinktank. "Merkel is caught between a rock and a hard place. On the one hand she would like to help Cameron out of the corner, as Germany wants Britain in rather than out. But in effect, her real game is the eurozone and therefore she will not keep Britain in the EU at all costs. And the rest of Europe will listen carefully to what she says in the British parliament."

Just over a year ago, Cameron's Bloomberg speech, which highlighted the need for European reform, found a surprising number of open ears in Germany, even among Social Democrats. "Not more Europe, but a better Europe" is a phrase you often hear in Berlin these days. German politicians openly talk of the need to reform the EU, something they share with few European countries apart from Britain.

The prime minister is being warned that he needs to be careful not to misinterpret Merkel's approach, as he did in late 2011 when he tabled a series of demands for the City of London at a Brussels summit. These were rejected by Berlin, forcing Cameron to veto the new eurozone fiscal compact. "Cameron's interpretation of Merkel's stance is partially based on a misunderstanding," said Stefan Kornelius, foreign editor of Süddeutsche Zeitung and author of an authorised Merkel biography. "He took her support of his stance in the EU budget debate as a statement of blanket support for Britain's renegotiation strategy. One priority for her speech will be to readjust expectations of what she can deliver."

While banking union and further economic integration in the eurozone will require some changes to the treaties, the German government would prefer to do so without having to reopen old agreements entirely: Vertragsanpassungen, "treaty modifications" rather than "treaty change", is the phrase her party uses. "Reopening the Lisbon treaty and having to get it ratified by all member states is the last thing Merkel wants," said Kornelius.

"Renegotiating treaties is definitely not an option," Gunther Krichbaum, the CDU chairman of the commission on European affairs, told the Guardian. "These treaties already are compromises – we'd end up where we started."

In any concession she can make to Britain on Thursday, Merkel will be restrained by her coalition agreement with the Social Democrats, who would never accept reopening the Lisbon treaty and deleting the phrase "ever closer union", as some British Conservatives would like to.

She will also have to take into account her own party's European manifesto, which is set to be presented at the CDU party conference in April. While the current draft hints at some common ground with the Tories over making it harder for EU migrants to access benefits, it also treats it as an issue to be addressed by national parliaments, rather than a problem that requires a wholesale revision of the freedom of movement principle.

Merkel and her speechwriters will be aware of the historic dimension of her visit. While British parliamentarians shouldn't expect rhetorical fireworks, it's possible she will add a personal flavour to her speech, as when she spoke in front of both chambers of the US Congress in 2009.

"Merkel is good at presidential speeches," said Möller. "There are plenty of things she can wax lyrical about without getting into tricky areas: the upcoming first world war centenary, the need for a more global outlook in the economy, the inspiring achievements of British parliamentary democracy." But whether that will be enough to appease Cameron's backbenchers is doubtful.