Jeremy Corbyn is expected to attack David Cameron’s negotiations of an “emergency brake” on benefits for new migrants as potentially discriminatory, and make a positive case for European migration ahead of the crunch summit on Britain’s EU membership this week.

Despite serious concerns among some of the shadow cabinet, it seems the Labour leader is determined to present an “alternative argument” that discriminating against workers from east European states is unfair and will do nothing to reduce migration levels.

It is part of a series of potentially contentious moves by Corbyn in the coming months designed to leave his political stamp on the party, including a new “Labour fiscal credibility rule”, under which the party would “guarantee that all cuts announced for this parliament could be reversed in full”.

Corbyn is planning to make his intervention on EU renegotiation during a visit to Brussels before the summit of member states on Thursday and Friday, where the prime minister will seek agreement on his renegotiation, including the idea of a four-year block on new migrants within the EU receiving in-work benefits.

The Labour leader will suggest that Cameron has been “playing at the edges” in his renegotiation, according to sources close to Corbyn, and will suggest a crackdown on the undercutting of wages by unscrupulous agencies paying eastern European workers below the minimum wage for jobs in the UK should have been a priority.

A high-level party source said there was a strong case for the alternative position to Cameron to be voiced by Corbyn, although it is understood that the speech is yet to be finalised.

It would be a high-risk manoeuvre which senior shadow cabinet ministers fear could allow the Tories and Ukip ammunition in vulnerable seats with which they would seek to portray Labour as out of touch on immigration.

It would also enrage many within the cross-party Britain Stronger in Europe campaign, which is backing the prime minister’s renegotiation, including the efforts to cut EU migration. An open letter signed by five big Labour beasts, former leader Lord (Neil) Kinnock, Margaret Beckett, Hilary Benn, David Blunkett and Jack Straw, says: “The conclusion of the current renegotiation will hopefully strengthen this relationship as we make the progressive case for Britain in Europe.”

However, the Observer has learned that Corbyn, and his ally the shadow chancellor John McDonnell, are determined to set out how they are a “genuine alternative” to the Conservatives in the runup to May’s council elections.

Alongside the controversial stance on EU migration, a major part of that process will be a new position on the economy. A document, entitled Labour’s Economic Credibility Strategy, seen by this newspaper, admits that “Labour suffers from a perceived lack of credibility on the economy”, but that there is now “close to a consensus among macro-economists that austerity represents a poor policy choice for governments”.

It adds: “The fact that austerity is widely perceived by much of the public as sound economic policy should tell us that credibility on the economy is less a question of technical expertise, as it is of having a convincing story.”

The document says Labour should “learn from the disciplined messaging which George Osborne has developed and the Conservative party has adopted: narrative repeatedly beats clever”.

It is suggested that a key part of Labour’s message should be a “Labour fiscal credibility rule” devised for the party by Professor Simon Wren-Lewis of Oxford University, that states that “obsessive Tory cuts” are killing the recovery and delaying a reduction in the fiscal deficit, but that investment prompts growth.

That message, it is claimed, would “allow [Labour] to address public concerns on the fiscal deficit, guarantee that all cuts announced for this parliament could be reversed in full and allow sufficient space for a programme of economic transformation”.

The document adds: “It is argued that properly and consistently presented, this would, over time, enable us to overcome a damaging perception of fiscal irresponsibility and allow us to move the debate on to our terms. This will require a broad consensus in the party, just as there is a consensus among the Conservatives on austerity.”

The Observer understands the shadow cabinet is due to discuss the new Labour rule this month. Sources close to McDonnell said there are plans to unveil it ahead of the budget.

The author of the credibility strategy writes: “Our major error on economic policy between 2010 and 2015 was to think that technical competence (backed up by economic theory) would be enough to secure us the public trust on the economy. This belief prevented us establishing a clear and consistent narrative for Labour’s approach to the economy.

“Instead, we offered multiple different stories and attempted to make short-term political capital when George Osborne missed his own self-imposed targets.”

An open letter

In the 1975 referendum we all campaigned against the UK remaining in what is now the European Union. Now, and for a long time past, it has been clear that Britain is stronger, safer and better off than we would or could be if pulled out of the EU.

Our concern then was that European membership would mean a one-way loss of sovereignty and investment. This has proved unfounded. The UK has the best of both worlds. We are part of an economic partnership with 27 other democracies, exercising full rights to determine agreed rules in the world’s largest single market. That participation has brought three million jobs linked to our trade in Europe, it attracts large investment, promotes growth, supports R&D and provides for employment rights that protect British workers.

At the same time we also have control of our vital national interests, notably our currency, borders, national security, defence, foreign affairs and justice. Britain’s voice on global matters, whether it’s about debt relief, peace-keeping or climate change, is amplified by being part of Europe and intelligence-sharing helps us to fight terrorism and other international crime.

The conclusion of the current renegotiation will hopefully strengthen this relationship as we make the progressive case for Britain in Europe. There is no alternative that would replicate all these advantages and leaving would be a huge risk to prosperity, security, essential influence and the opportunities of future generations. The EU is plainly not perfect and improvement is always worth making, but the benefits far outweigh the costs.

Forty years ago Labour was split on Europe. Having suffered the results of that and recognised the realities of the modern world, we changed policies and are now the most united major party on this issue. That, and the practical interests of the British people, will be the basis of our campaign to Remain in this referendum.

Signed, Margaret Beckett, Hilary Benn, David Blunkett, Neil Kinnock and Jack Straw