BRUSSELS (AFP) - European Commission head Jose Manuel Barroso paid tribute on Monday to former British premier Margaret Thatcher's "contributions" to the growth of the EU, despite her reservations about its merits.

Expressing his "deepest regrets" to the British government after Mrs Thatcher died aged 87 following a stroke, Barroso said she had been "a circumspect yet engaged player in the European Union" who "will be remembered for both her contributions to and her reserves about our common project."

"I was deeply saddened to hear the news of the death of Baroness Thatcher ... without doubt a great stateswoman," former Socialist Portuguese prime minister Barroso said in a statement.

"She signed the Single European Act and helped bring about the Single Market. She was a leading player in bringing into the European family the Central and Eastern European countries which were formerly behind the Iron Curtain.

"As you remember, Britain under Mrs Thatcher's leadership was very supportive of the enlargement of the European Union."

But he also made clear reference to present difficulties between EU partners and Mrs Thatcher's heir in today's Conservative Prime Minister David Cameron - who wants to stage a referendum on membership terms he aims to renegotiate in the hope of reducing Brussels' powers.

"Her legacy has done much to shape the United Kingdom as we know it today, including the special role of the United Kingdom in the European Union that endures to this day," Mr Barroso said.

Mrs Thatcher famously secured a landmark budget rebate from the EU in 1984 that rankles French and German counterparts to this day, especially with Cameron leading so far successful efforts to cut back Barroso's one-trillion-euro 2014-20 plan.

The German Socialist head of the European Parliament, Martin Schulz, also hailed "a figure of historic significance" while alluding to "clear political differences."

He said Mrs Thatcher "at the beginning of her tenure was a committed European, signing and pushing for the single European act which transformed the EU single market.

"No matter whether one agrees with her policies or not, Margaret Thatcher showed that politics still has the capacity to be a force for change," he added.

"My thoughts are with her family and friends."