Tory party officials refused to meet the Electoral Commission in the course of the election watchdog's 14-month investigation into £5m it received from a company owned by Lord Ashcroft.

The commission ruled that the donations did not breach election laws, but it noted that Tory officials declined invitations to explain the details of arrangements in person.

The revelations pose new questions for the Conservatives over their acceptance of the money, which has been ploughed into marginal seats ahead of the election.

Senior figures will be called to give an account of their decision to accept the cash.

In its report, the commission says it cannot conclude whether the Tories complied with their duty to be certain about the identity of the donor because it was unable to question party officers directly.

It notes internal party emails – and sections of Ashcroft's book Dirty Politics, Dirty Times, which suggest the donations were from him rather than his company but it could not make a decision without direct evidence.

The commission also expressed strong frustrations at its inability to compel either the Tories or Ashcroft to give evidence in person and the fact that it could not order Bearwood to release any private documents.

The commission dismissed two charges: that Bearwood was not functioning as a proper company in the UK and that it was in fact a proxy for Ashcroft to make donations. The decision will be a huge relief to the party, which faced paying back £5.1m if the donations were ruled impermissible.

Jenny Watson, chair of the commission, announcing that Bearwood had been cleared, said: "We had concerns, based on some of the evidence, about the degree of certainty within the Conservative party about the identity of the donor, but based on the evidence before us, the commission considered that there was not sufficient evidence to conclude that the party was uncertain about the identity of the donor when they accepted the donations.

"We have asked to meet party officials to ensure that they are clear about their responsibilities for complying with this aspect of the law."

The commission's report describes its powers as "limited", saying it could not require the donor to release documents. "The commission currently has no power to require potential witnesses to attend for interview. Within the limits of its current powers, the commission conducted a thorough investigation."

John Mann, the Labour MP who made the complaint, said: "The report opens up more questions than it answers. There's no surprise that without the powers they needed they couldn't establish the facts."

A Conservative party spokesman said: "Following an 18-month investigation, the Electoral Commission has definitively concluded that donations made by Bearwood, the company in which Lord Ashcroft has an interest, were legal, permissible and correctly reported. It has now been put beyond doubt that donations from Bearwood were entirely legitimate.

"Separately, we also know that Lord Ashcroft is non-domiciled, putting him in exactly the same position as Labour and Lib Dem donors such as Lord Paul.

"This means the Conservative party's clean bill of health with the Electoral Commission remains fully intact."

The spokesman went on to say that continued attacks on Lord Ashcroft were politically motivated.

The decision and the commission's apparent frustration will prompt concerns that the investigators could not unravel Ashcroft's complex financial affairs.

Mann's initial complaint claimed that Bearwood was in breach of electoral law because it was not trading in Britain when it gave money.

Donations from overseas companies are prohibited by law. The ultimate source of the Ashcroft millions that have helped bankroll the Tories in the past appears to be Belize, the Caribbean tax haven that the billionaire has claimed in the past to be his home.

But the route that the money follows on its 5,200-mile journey from the impoverished country to Conservative HQ – and then out to Britain's marginal constituencies – is highly complex.

In recent years, the tycoon's donations to the party have been made by Bearwood Corporate Services (BCS), a company registered in the UK and with a registered office at the offices of its auditors, BDO Stoy Hayward, in Southampton.

During the year ending March 2006, BCS received £4.79m in cash for shares that were bought by its holding company, Bearwood Corporate Holdings.

Bearwood Holdings had received that money by selling shares in itself to another company, Astraporta UK, for £5.54m.

Astraporta, in turn, appears to have received its funds, around £6m, by selling shares to a company registered in Belize called Stargate Holdings. Where Stargate receives its funds is unclear. It is registered offshore – at a registry controlled by an Ashcroft company. When the Guardian visited the registry's offices in Belize City to inquire about Stargate, a registry official said: "You will never know who owns Stargate."

Astraporta and Bearwood Holdings were put into liquidation last year and were formally dissolved on Monday, just as Ashcroft was making his announcement that he was a "non-dom".

This morning it was also announced that Ashcroft will be invited to appear before MPs to explain his version of how he came to be awarded his peerage in 2000 on the basis of a promise to become a permanent resident in the UK and how he subsequently renegotiated that deal to avoid paying taxes on his international earnings.

The Commons public administration committee is to conduct an inquiry before the election, starting with a hearing on 18 March at which senior civil servants will also be quizzed on their role in the affair.