European leaders have struck a broad climate change pact obliging the EU as a whole to cut greenhouse gases by at least 40% by 2030.

But key aspects of the deal that will form a bargaining position for global climate talks in Paris next year were left vague or voluntary, raising questions as to how the aims would be realised.

As well as the greenhouse gas, two 27% targets were agreed – for renewable energy market share and increase in energy efficiency improvement. The former would be binding only on the EU as a whole. The latter would be optional, although it could be raised to 30% by a review in 2020.

“It was not easy, not at all, but we managed to reach a fair decision that sets the EU on an ambitious but cost-effective climate path,” Herman Van Rompuy, the president of the European Council told a press conference in Brussels.

“This package is very good news for our fight against climate change,” the European Commission president, Jose Manuel Barroso, added. “No player in the world is as ambitious as the EU.”

With an eye on the haggling expected ahead of a global climate summit in Paris next year, the EU’s climate commissioner, Connie Hedegaard, said the agreement was an important step for the whole world. She said: “We have sent a strong signal to other big economies and all other countries: we have done our homework, now we urge you to follow Europe’s example.”

But a clause was inserted into the text that could trigger a review of the EU’s new targets if other countries do not come forward with comparable commitments in Paris.

The Brussels summit was dominated by arguments over energy savings and climate policy, with countries from Poland to Portugal pleading special circumstances and threatening to veto any breakthrough unless their demands were met.

David Cameron was keen to minimise any perceived loss of UK sovereignty over energy policy, for fear of further exposure to attacks from the Eurosceptic wing of his Conservative party and Ukip. The prime minister won a battle to keep policies aimed at boosting renewables and saving electricity voluntary for member states.

“It’s important that you’ve got flexibility over your energy mix,” said a Downing Street spokeswoman. Cameron had hoped to cut the energy efficiency figure to 25%, but was prepared to accept 27% as long as it was not binding on Britain.

Portugal attained a non-binding objective that 15% of the bloc’s energy be transportable via cross-border connections by 2030, with an invitation to the European Commission to make concrete proposals for project financing from the EU budget.

Danish concerns were addressed with the introduction of a “cap and trade approach” to sectors previously considered outside the bloc’s carbon market such as agriculture, buildings and transport – which alone represents 31% of the bloc’s emissions.

Poland, heavily dependent on coal-fired energy production, threatened to block the deal unless the costs to its economy and industry were discounted by €15bn-€20bn (£12bn-£16bn) between 2020 and 2030, under a complicated system of concessions from the EU’s carbon trading system.

Concessions granted to Poland will allow it to continue reaping hundreds of millions of euros in free allowances to modernise coal-fired power plants. Of eight EU nations eligible for the free allocations, Poland claimed 60% of the total up until 2019.

A poll by TNS and YouGov for the online activist group Avaaz late last week found that 56% of Poles thought that EU financial support for energy should back clean energy rather than fossil fuels.

“It’s scandalous,” said Julia Michalak, a spokeswoman for Climate Action Network Europe. “A continuation of free emission permits for Poland’s coal-reliant energy system would be a grave mistake. Leaders who came to Brussels to agree new historic climate goals, are actually discussing whether to hand out money to Europe’s dirtiest power plants.”

Intense bilateral discussions between Cameron, the German chancellor, Angela Merkel, and other EU leaders over the last week tried to find ways of placating the Poles, who kept open their option of vetoing the summit outcome until the end.

The anticipated 40% greenhouse gas cut by 2030 would be measured against benchmark 1990 levels. That figure is to be binding on the EU and the minimum level achieved, with Germany and Britain happy to agree a higher figure.

Tony Robson, the CEO of Knauf Insulation – a leading insulation firm that had threatened to divest from Europe unless firm energy saving targets were announced – said that the 27% figure for energy efficiency improvement was “no better than business as usual” in an open letter to EU leaders.

A 27% target “sends a strong signal to the energy efficiency industry to ‘leave Europe and make your investments elsewhere’”, he wrote.