A City trader has won the backing of the equality watchdog in her £1.35 million claim against a bank that she accuses of discriminating against her when she returned to work after having a baby.

Katharina Tofeji, a senior sales dealer, is seeking to overturn a tribunal ruling that she did not suffer sex discrimination when she was prevented from returning to her old job at the investment bank BNP Paribas. She also claims that she was exposed to a culture of sexism at the bank, where colleagues made negative comments and one placed a bet on how long she would last back at work.

Yesterday the Equal Opportunities Commission (EOC) announced that it would back her appeal, to clarify the legal rights of working women who leave their jobs to have a baby.

Miss Tofeji, 38, had worked with the investment bank for five years from October 2000 until her resignation in June last year. She argues that, after returning from a year’s maternity leave, she was placed in a vulnerable position and at a disadvantage because all her clients had been reassigned and a male colleague had been assigned permanently to her team.

Nor were there any “immediate plans to return the 29 clients she had successfully built up during her time at the bank”, the EOC said. “Instead, she was told to justify the return of any of her clients [to her],” said the commission. She was also refused flexible working hours (she wanted a four-day week) and experienced “a hostile reception” from colleagues.

Miss Tofeji, who is Austrian-born and lives in Brentwood, Essex, resigned. In June this year a tribunal dismissed her £1.35 million constructive dismissal claim. It ruled that she was not wrongfully dismissed, nor treated less favourably than male colleagues.

Nor, it added, did the bank breach regulations by “allegedly failing to allow the claimant to return to the job in which she was employed before her absence on maternity leave or to another suitable and appropriate job”.

Miss Tofeji has not worked since she resigned from her £70,000-a-year job, as a result, she says, of continuing depression.

Her appeal will argue that the tribunal decision wrongly compared her treatment with how she would have been treated as a man on leave for a similar period of time.

But the commission said yesterday: “Pregnant women are in a unique position, so it is not the correct approach simply to compare the situation to someone who has not been pregnant.”

This was the important legal issue at the heart of the appeal. The right test was whether women had been disadvantaged as a result of taking maternity leave. As many as 30,000 women a year lost jobs because of pregnancy or maternity leave, it added.

Jenny Watson, who chairs the EOC, said: “We know from our research that many women experience discrimination upon return from maternity leave and suffer considerable stress and hardship as a result. Miss Tofeji’s case provides yet another reminder of how easily the thin veneer of equality can crack when working women start a family.”

Miss Tofeji said that she had been very disappointed with the tribunal decision and was delighted that the EOC was backing her appeal. She said: “It is important not just for me but for all other working mothers who find themselves being forced out of jobs they love and are successful at, once they decide to have children.”

A date is expected to be set for the appeal hearing within the next few weeks.

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