Northern Ireland’s former first minister Arlene Foster has told the inquiry into the green energy scheme that resulted in the collapse of the power-sharing government that she deeply regretted its spiralling costs.

The renewable heat incentive (RHI) programme was championed by Foster when she was the devolved enterprise minister but its costs ran into hundreds of millions of pounds.

Foster told the public inquiry into the scheme that she believed Sinn Féin protests over RHI costs were merely an excuse to bring down the power-sharing executive at the start of 2017.

The late Martin McGuinness resigned from his post as deputy first minister after Foster refused to step aside temporarily as first minister to allow the inquiry to run its course. The Sinn Féin politician’s resignation triggered the collapse of the power-sharing government more than 14 months ago.

Foster denied she was mainly responsible for RHI and told the inquiry that the scheme wasn’t a personal priority of hers. Foster said she was “personally and politically” sorry as to how the RHI scheme turned out.

The inquiry, being held at Stormont, is seeking to establish exactly why there were no cost controls imposed on the scheme before its implementation.

Foster said the power-sharing coalition in Belfast had been under pressure from 2010 onwards to create a cleaner, greener renewable energy scheme.

“If we had decided as 3% of the population [of the UK] that we were not going to do anything on renewable energy I would have come under political attack from other parties,” the Democratic Unionist party leader told the inquiry.

Foster said the power-sharing executive had to hit a target of having 4% of Northern Ireland’s energy renewable and green by 2020. She accepted, however, that the department she headed should have taken up a number of alternative renewable energy initiatives that were operating in Britain.

“It is a regret that we didn’t buy into it and, looking back now, that’s what should’ve been done,” Foster said.

During her cross-examination at the inquiry on Thursday, its chair, retired judge Sir Patrick Coghlin, questioned Foster over claims made by her one-time special adviser Andrew Crawford that he did not read all crucial reports on the scheme in detail.

Given the complexity of RHI and its mounting costs, Coghlin told Foster: “That just seems to me to be perhaps not the highest standard of government practice.” He said this concerned him as there was therefore no proper record of what was discussed in the department and what decisions had been made.

Crawford revealed earlier to the inquiry that he had established a system with Foster as enterprise minister where he would send her yellow Post-it notes that were later destroyed. He said these messages would not have been formally recorded in the system. Foster insisted at the inquiry that these notes were “innocuous”.

The so-called “cash-for-ash” scandal has cost hundreds of millions of pounds. RHI offered huge financial incentives to farms, businesses and other non-domestic consumers to use biomass boilers that mostly burned wood pellets as well as solar and thermal energy and heat pumps. The scheme paid more to users of boilers than the cost of the fuel needed to run them, allowing applicants to profit from using the boilers.

The whistleblower who exposed the scandal alleged that the RHI scheme was being abused and that one farmer had made £1m by renting an empty shed he had claimed was for the biomass boiler.

