The eminent scientist Sir William Stewart has broken his silence on events that led to his quitting as chair of Scotland's most financially troubled trust.

He told the Scottish Parliament's audit committee that it was difficult to meet politicians' demands not to rock the boat while trying to follow orders from the health department to stay in budget.

MSPs who sat through the evidence-taking session heard conflicting accounts from current and former managers in Tayside as they tried to untangle why finances had got out of control.

But Sir William said: 'We faced a dilemma. On the one hand, the management executive was telling us to ensure financial targets were met. On the other, we were being told not to do anything politically embarrassing. That meant no redundancies and making sure everybody had to be treated. '

He added: 'It was like being asked to drive from Dundee to London in two hours, told to observe the speed limit and you have only enough fuel to reach the border. '

Sir William quit last year before the publication of a report by the taskforce sent in to Tayside University Hospitals by Scottish health minister Susan Deacon. The report found evidence of management failings and was followed by an investigation by the auditor general.

The investigations discovered that the new trust, which was formed in 1999 from three other trusts, including Perth and Kinross and Dundee Teaching Hospitals, very quickly forecast a deficit of a probable£12m. They also found that the trust had used non-recurring funds to meet recurring costs - a practice which Sir William said was common across Scotland. The ensuing financial problems have led to a cost-cutting programme and a likely cumulative deficit of between£21m and£23m by next year.

Trust chief executive Paul White, who has just been appointed chief executive of Barts and the London trust, faced tough questioning from MSPs about when he realised there was an impending problem. He has consistently said that he was 'surprised' by the magnitude of the deficit when it emerged at the end of the new trust's first quarter.

But Philip Colville, former finance director of Dundee Teaching Hospitals, said that it had been in the public domain from the previous September that the deficit could reach£9m.

He told MSPs that he had himself mentioned the financial problems to Mr White at his interview.

Mr White retorted that Mr Colville had not been on his interview panel and said he had thought predictions of a£20m deficit were 'hearsay'.

The audit committee will now hear from other figures, including the former chief executive of the NHS in Scotland, Geoff Scaife.