Scottish Liberal Democrat Alex Cole-Hamilton said MSPs and all other elected members should be subject to a Protecting Vulnerable Groups (PVG) check, of the type teachers, social workers and others who have regular contact with children and vulnerable adults need to pass.

The checks, which were introduced in 2011, would ensure politicians have no history of harmful behaviour.

His call for tougher regulation of MSPs came as Scottish Labour leader Richard Leonard demanded that Nicola Sturgeon reveal why she allowed Mr Mackay to resign rather than sack him, after it was revealed he had bombarded a 16-year-old boy with unwanted text messages, and as the SNP deputy leader Keith Brown said it would be difficult for Mr Mackay to continue as an MSP.

Derek Mackay resigned as finance secretary last week and was suspended by the SNP

Last night Mr Cole-Hamilton said that, as MSPs came into unsupervised contact with young people and vulnerable adults, they should not be “exempt from the checks that would reasonably happen in any other workplace”.

He said the new Disclosure (Scotland) Bill, which is due to go before Holyrood’s education committee for scrutiny next month, should be amended to ensure elected representatives undergo a PVG check. He said: “It’s an important part of the job to engage with people of all ages so we can represent them and their views. Parents should know their children are in the presence of someone who can be trusted and has been checked up on. But those assurances simply don’t exist at the moment and that’s wrong.

“I have always been conscious that with elected office comes a huge power imbalance. We know that people can use their status to manipulate, target and exploit. People need protecting from that and this needs to be done through law.”

He added: “Parties each have their own candidate approval processes, but none will offer the same level of protection as a PVG certificate. The prospect and knowledge of this extra independent check will also serve the purpose of deterring people who would be inappropriate from standing for public office in the first place.

“I have been exploring this proposal for months. This isn’t in response to any single incident. Rather, the idea that winning an election should exempt you from rigorous checking of your suitability to be left alone with children or vulnerable adults is outdated and outright dangerous.

“Being elected to represent people is a privilege. Those who are in office cannot be exempt from the checks that would reasonably happen in any other workplace.

“I hope all parties will get behind my proposal and see the value in increasing the level of protection given to children and vulnerable adults.”

The Scottish Government’s children’s minister, Maree Todd, has previously said the new Disclosure Bill should focus less on job titles but where there is a “power imbalance” she was “more than happy to assess whether parliamentarians fall into that category”.

The issue has become more acute after Mr Mackay’s texts to the schoolboy, whom he first approached by Instagram messaging “out of the blue”. In the messages Mr Mackay suggested the rugby-mad boy be his guest at a Scottish Parliament reception for Scottish Rugby and encouraged him to deliver campaign leaflets for him and then go for dinner.

Mr Mackay resigned last Wednesday night when his messaging was revealed, the day before he was due to deliver the Scottish Government’s latest Budget. The boy at the centre of the allegations has spoken to Police Scotland and the force is believed to be assessing the information.

However, as a result of resigning, rather than being sacked from his government post, he is still entitled to pick up a “resettlement grant” from the Scottish Parliament of nearly £12,000. Now Mr Leonard has written to the First Minister asking her to explain why she allowed Mr Mackay to resign and not immediately dismiss him.

Mr Leonard said: “It is deeply concerning that such a senior member of this parliament was not immediately dismissed upon admitting that the allegations against him were true.

“The fact that his behaviour was directed towards a teenage schoolboy should have been enough for the First Minister to take decisive and fast action. It is now vital that the full circumstances leading to Derek Mackay’s resignation are revealed publicly.

“The contact Nicola Sturgeon had with Mr Mackay after she was made aware of the allegations should be shared immediately.”

He added: “The conduct that the cabinet secretary has admitted to would, in my view, merit a dismissal from office by the First Minister rather than allowing a resignation.”

Mr Brown said the SNP and government had reacted “seriously” to the matter but that, while they could suspend him pending an investigation, Mr Mackay would have to come to his own decision” on whether he should stand down from Holyrood.

He added: “It’s very difficult to see how he can continue.”

Derek Mackay scandal triggers call for checks on all politicians