A Holyrood committee has summoned Ms Hyslop to state the reasons for signing off the £150,000 package, as a former adviser to Alex Salmond abandoned her attempt to become an SNP MSP following a cronyism row over the award.
Jennifer Dempsie was criticised after it was claimed she used her ministerial contacts to help arrange the funding package to help the music festival relocate.
Ms Dempsie, who worked as a project manager for promoters DF Concerts Ltd, brokered an initial meeting between T in the Park and DF Concerts boss Geoff Ellis and culture secretary Ms Hyslop to discuss the funding.
Ms Hyslop has faced claims by opposition parties of breaking the ministerial code and a cross-party group of politicians requested the right to question the minister over the issue.
The cabinet minister was cleared of any wrongdoing after an internal investigation - but watchdog Audit Scotland has announced its own inquiry and she will now be called to appear before MSPs to explain her actions.
Holyrood’s education and culture committee yesterday agreed to ask the minister to appear before it later this month to account for the taxpayer-funded grant to the festival.
Committee convener Stewart Maxwell, confirming the move, said: “During the discussion of the committee’s work programme, which included the issue of T in the Park, it was agreed that it was important to understand the decision-making process involved in awarding funding to T in the Park.”
While the Scottish Government was assessing the state aid deal, Ms Hyslop publicly endorsed Ms Dempsie’s bid to become an MSP, writing on her Facebook page: “Good luck Jen, you’d make a formidable SNP candidate and a great asset to the Scottish Parliament.”
However, Ms Dempsie announced she would abandon her campaign to become an SNP list MSP at next year’s Scottish Parliament elections.
In the posting on her social media page, Ms Dempsie said: “I have decided I will not be taking forward my nominations across the Highlands and Islands to be a list SNP MSP for the region.”
Ms Dempsie, the partner of Nationalist Westminster leader Angus Robertson, was a special adviser to First Minister Mr Salmond from 2007 to 2009, then worked for the SNP for four years before starting work as a public relations consultant.
The T in the Park funding was justified as support for the festival’s relocation from its traditional home in Balado in Perth and Kinross, following concerns about an underground oil pipeline, to Strathallan Castle.
Conservative culture spokeswoman Liz Smith, welcomed the decision to summon Ms Hyslop to appear before the committee on a date likely to be before the end of this month.
She said: “This is very welcome news as it is imperative that the culture secretary puts the full facts on the table.”
A Scottish Government spokeswoman said: “Culture secretary Fiona Hyslop looks forward to giving evidence to the education and culture committee.”