The head of the Home Office has praised the secretary of state following her decision to stand down.

Permanent secretary Sir David Normington said Jacqui Smith had shown "exceptional leadership" during the her two year stint as home secretary.

Smith is one of a number of cabinet ministers who have announced that they do not want to continue in the government, ahead of a major reshuffle.

The home secretary had told Gordon Brown at Easter that she wanted to stand down, following embarassing revelations about x-rated television channels that were claimed on her expenses, to focus on her family and her Redditch constituency. She had intended to make the announcement after the European elections, but the news was leaked just days before.

She wrote to Home Office civil servants, stating that she had "been lucky to work alongside the best civil service in the world". She was also pleased "that the Home Office is now seen as a strong and effective department where people are increasingly proud to work – and they should be".

But "undeserved intrusion and pressure" on her family "makes it difficult for me to devote the time and energy necessary to do myself – and more importantly a ministerial job – justice", she added.

Sir David said Smith had allowed the department "to come out of our previous difficulties". The department was famously described as "not fit for purpose" by Smith's immediate predecessor, John Reid.

Smith had allowed staff to regain their confidence, Sir David said: "In private she was always challenging us to improve; in public she was always supportive. We could not really have asked for more."