Many women feel marginalised at work after announcing their pregnancy

Other pregnant workers face demotion, hostility, pay cuts or are forced to work in unsafe conditions, the Equal Opportunities Commission (EOC) added.

A further study from shop workers union Usdaw found ignorance and lack of awareness was putting pregnant women and their unborn babies at risk.

The investigations have been published to mark National Pregnancy Week.

'Tip of the iceberg'

Many women told us they felt undervalued, ignored, humiliated, even made to feel guilty about being pregnant John Hannett, Usdaw 'My pregnancy cost me my job'

"Some employers knowingly flout the law but many simply don't know what their responsibilities are or what help with costs is already available from the government," EOC chairwoman Julie Mellor said.

In its report, "Tip of the Iceberg" the EOC calls for government action to deter discrimination against pregnant women in the workplace.

Among the proposals put forward to resolve pregnancy discrimination were:

• A written statement of rights and responsibilities be given to every pregnant woman and her employer
• A practical toolkit for employers to clarify the "complicated jigsaw" of existing law
• Extending the three-month time limit for filing pregnancy-related tribunal claims as many women find it difficult to cope with the process near childbirth
• A national family strategy to deliver a coherent framework of parental employment rights and services

The EOC said that with women making up almost half of the UK's workforce - and 350,000 working women becoming pregnant each year - action was vital.

"Women ... make a vital contribution to the success of individual companies and the economy," Ms Mellor added.

"We cannot afford to lose their skills and experience. At a time when the birth rate is falling, we also rely on the next generation for our prosperity and welfare, including our pensions."

'Marginalised'

In a separate report, Usdaw found two thirds of retail workers suffered a negative change in attitude from their employer after they announced they were having a baby.

One in four of the 1,200 pregnant women polled by Usdaw said they were made to feel "marginalised", while a similar number said they were not given a suitable uniform to wear.

I had no risk assessment. My manager never asked me how I was coping. I collapsed at work and was admitted to hospital twice Nottingham shop worker Maternity and paternity rights explained

Many workers had to alter clothing themselves - adding zips or elastic, or borrowing clothing from larger co-workers.

"Many women told us they felt undervalued, ignored, humiliated, even made to feel guilty about being pregnant," Usdaw general secretary John Hannett said.

"Women who asked for help were told to stop complaining, repeated requests for maternity uniforms fell on deaf ears and women were excluded from decisions on the basis that they were pregnant," he added.

'I collapsed'

One shop worker from Nottingham told the union that she had been forced to lift trays of meat, fish and cheese - the latter as heavy as 20kg - while working on a deli counter.

"I had no risk assessment. My manager never asked me how I was coping. I collapsed at work and was admitted to hospital twice," she told the survey.

The union also found that many women were refused time off for antenatal appointments, or were forced to work back their time.

Many women return to work on a lower grade, Usdaw found

Among women who made an employment tribunal claim, almost one quarter were dismissed within hours or days of telling their employer.

Furthermore, 20% of women who returned to the same employer after maternity leave went back to a lower grade or level of job, Usdaw found.

On the other hand, more than one-third of employers felt pregnancy was an "undue cost burden" - but some were unaware of their right to reclaim Statutory Maternity Pay.

Claim costs

While some companies may claim pregnant workers are an unnecessary expense, they should bear in mind the latest figures from the Equal Opportunities Review (EOR).

Unlawful discrimination cases - whether concerning race, disability or sex - cost business more than �4m in settlements last year.

Awards, which averaged �11,000, also showed an upward trend, it said.

The EOR also found that half of the sex discrimination cases involved pregnancy or childcare.

But both Usdaw and the EOC did emphasise that there were many examples of companies that did support their pregnant employees.

"Where we found good practice, Usdaw will be working with employers to help encourage its spread, but equally we're already approaching retail employers to ask them to work with Usdaw to tackle the many problems highlighted by this research," Mr Hannett said.