The Tory leader sought to outflank Labour on family policy and the work-life balance by calling for a change of culture in the workplace to put more emphasis on quality of life.

Mr Cameron told the Equal Opportunities Commission that bosses should look more favourably on flexible working for all employees, particularly women with children, people over normal retirement age and the disabled seeking a return to work.

The Tory leader stressed the benefits of helping staff juggle their work and family lives better, including a happier, more productive workforce.

With Britain facing increased competition from countries such as Indian and China, it must be a "trailblazer of innovation in the way we work".

"Human capital is the natural resource of 21st century Britain," he said. "Flexible working is good for companies, good for the economy and good for society as a whole."

The legal right to request flexible working is currently available for parents with children who are under six or disabled children under 18.

From April, the right was extended to people who needed to alter their working hours to give them time to care for an adult.

This includes people who, for example, need to get away from work to look after parents suffering dementia or family members who are disabled or ill. Mr Cameron said the Conservatives believed that the option of flexible working should go as wide as possible.

"We will extend the legislation on the right to request to all parents."

He said a Tory government would also work with business to help firms implement existing regulations, and pledged to ensure that the public sector became a world leader in providing flexible working opportunities.

The Tory party was leading by example, he said, and as an employer had introduced the universal right to request flexible hours, and was encouraging more home working.

Under current rules, there is no compulsion for employers to agree to flexible working requests.

All an employer is legally bound to do is give due consideration to the request; they can turn it down flat, without giving a reason.

What is more, even if the employee claims that their request has been turned down unfairly an industrial tribunal is barred from probing the business reason.

John Cridland, deputy director of the CBI, the employers' organisation, said firms had embraced the concept of flexible working and almost 95 per cent of all requests to work flexibly were being granted.