Fears of political interference and a complex set of ill-defined market reforms are starting to scare investors from the UK, executives from some of the country’s biggest energy companies told MPs on Tuesday.

“This country used to be seen as a fantastic energy market to invest in,” said Keith Anderson, ScottishPower head, whose group is a leading UK wind farm investor, owned by Spain’s Iberdrola, and operates several coal and gas power stations.

That was because “everyone had absolute faith and trust” that policy was drawn up according to independent evidence-based research, he told a hearing of the energy and climate change committee in Westminster.

There had been an “awful lot of noise” in the past six months about “political influence” in the latest government review of renewable energy support, he said, a reference to recent reports that George Osborne, the chancellor, was backing calls from some Conservative backbenchers to cut wind farm subsidies.

“That detracts and damages confidence to invest in this market,” said Mr Anderson, adding that it could make investors worry that political pressure would also affect sweeping electricity market reforms that parliament is due to debate later this year.

After much consultation, the government finally published a draft energy bill last month aimed at spurring the £110bn of investment ministers say is needed over the next decade to upgrade the UK’s ageing electricity infrastructure.

But Mr Anderson and executives from some of the other so-called Big Six energy companies said the draft legislation still lacked a large amount of critical detail, which worried investors. Ian Marchant, SSE chief executive, said he did not even think the legislation was necessary.

“We have now got a hiatus in investment,” said John McElroy, director of policy and public affairs at RWE Npower, whose German parent, RWE, ditched plans to build new nuclear reactors in the UK with partner Eon earlier this year.

Mr McElroy pointed to Germany as an example of a country that had made investment in offshore wind in particular attractive to investors. Others said France had made similar progress.

The measures in the energy bill, which one committee MP described as “comically complex”, include incentives to invest in low carbon energy that involve long-term contracts for which a set price is guaranteed.

But there was still uncertainty about both the price and the creditworthiness of the contracting counterparty, several executives said, which made it difficult to know what sort of returns investors could expect.

Vincent de Rivaz, the chief executive of EDF Energy, was one of the most supportive of the six executives to give evidence, describing the energy reforms as “an important milestone”.

He said EDF, which is due to make a final investment decision on building new UK nuclear plants at the end of this year, also needed clarification of several aspects of the draft bill, including the precise details of the long-term contracts, but he was confident this would happen.

Other executives also questioned the proposed “capacity mechanism”, a measure designed to compensate owners of coal and gas plants to provide back-up power even when the growth of low carbon power made investment in such plants riskier.

“The government has created a known-unknown,” said SSE’s Ian Marchant. “They have said there will be a capacity mechanism but not what it will be.”