President of the European Commission says free movement is open to abuse but that he will not compromise on citizens' rights

José Manuel Barroso, the president of the European Commission, will on Friday acknowledge that the free movement of people across the EU has put "unintended strains" on public services and is open to abuse.

In a move to show that Brussels understands the concerns raised in Britain, Barroso will say in London that the commission has recently clarified anti-abuse rules to crack down on sham marriages which allow non-EU citizens to claim benefits as a family member.

But the commission president will make clear in a speech to the London School of Economics that he will not compromise on the right of all EU citizens to move across all 28 member states – one of the four founding pillars which guarantees the free movement of labour, capital, goods and services.

The speech by Barroso came as Whitehall sources confirmed that a government report on free movement was delayed on Thursday after the home secretary Theresa May complained that it did not contain enough evidence on "benefit tourism".

Officials told May that there was limited evidence of the practice in which citizens of less affluent EU member states travel to richer ones, such as Britain, simply for the purpose of claiming benefits. May then asked for polling on public concerns about the abuse of benefits which raised alarms in Whitehall.

The report on free movement was meant to be the ninth of a series of reports from the government's "balance of competences" review – an assessment of the impact of EU regulations – published on Thursday. One Whitehall source said: "These reports are meant to be sober and based on facts. The foreign office is in the lead and is running the process properly. But Theresa May is trying to work outside those procedures and wants to produce a report that fits her already established prejudices."

The European Commission has angrily rejected home office claims about "benefit tourism" after May's office leaked a draft version of the report which raised the prospect of imposing a 75,000 cap on non-UK EU citizens settling in Britain. It also took exception when the government seized on a report in the Sunday Telegraph last year which said that "600,000 unemployed European Union migrants are living in Britain". The figure covered "economically inactive" people, such as students and pensioners or non-working parents. The number of people claiming jobseeker's allowance was around 10% of the total – lower than the ratio for UK citizens.

Barroso will acknowledge the concerns about the freedom of movement of EU citizens. He will say: "I am very much aware that free movement may put unintended strains on local communities and services and in some cases there has been abuse. There is a lot that national governments can do to tackle abuses if and when they occur.

"European rules include firm measures to tackle fraud. The European Commission is eager to help. We can provide financial support for integration through the European Social Funds. And we have clarified anti-abuse rules, for instance on sham marriages."

But he will insist that there can be no compromise on free movement. He will say: "We are tough on abuse. At the same time, we remain absolutely firm on the principle of free movement rights … you cannot have a single market without free movement of European citizens.

"Those who move to take up work tend to put way more into the tax system than they take out in benefits … can you imagine a situation where goods, capital and services could move from New York to California, but people could not?"

Amid nerves in Brussels that Britain is in danger of walking to the EU exit door, the anglophile Barroso will say the EU does not impose "diktats".

He will add that Britain has played a decisive role in promoting reform in the EU. He will say: "Without the UK, Europe would be less reform-driven, less open, and less international. Less effective as a tool for managing and benefitting from globalisation. For a country so open to the world, it's a paradox that the UK remains so torn on Europe.

"The European Union is not some diktat forced upon Member States. It is created by Member States to work with and for them and promote our shared European interest at home and in dealing with the rest of the world. The UK, which is very good at selling its views and promoting its interests in the European Union, is a lot stronger as a result of it.

"When it comes to the British question...the British people can decide for themselves. My very personal, very strong conviction is that the EU is better off with the UK, and the UK is better off in the European Union."