The TUC is mounting a legal challenge to the government's rules on parental leave, after the prime minister's wife said they were not properly implemented.

Mrs Blair, who uses her maiden name Cherie Booth in her work as a leading employment lawyer, had been employed by the TUC to offer legal opinion. She said the government was in breach of European law when it excluded millions of parents from new rights to take extra leave.

The directive, which offers 13 weeks' unpaid leave, came into force last month for parents of children born on or after 15 December until they reach the age of five.

Confirming its decision, TUC general secretary John Monks said: "It gives us no pleasure to be taking the government to court."

However a spokesman for the Department of Trade and Industry said: "We are satisfied that we are not in breach of the directive and that we have taken appropriate legal advice."

In the UK, the new paternity leave law was introduced with a cut-off date.

Normally any parents with a child under the age of five should be entitled to the leave, but in the UK, only children born after 15 December 1999 are eligible.

Unions and other campaign groups say this excludes 3.3m people who already had children under the age of five.

A spokesman for the Manufacturing Science and Finance union, which has complained to the European Commission about the cut-off date, said he was "delighted" with the legal opinion.

"It bears out what we have been saying to the government," he said. "We hope the government will now act quickly to put right its mistake and give 3.3 million parents the extra leave."

And GMB union spokesman said Mrs Blair's advice was a "significant development", adding: "Our view has always been that the government would find itself in the dock over this policy when it should have been receiving plaudits.

"We now urge ministers to think again and end this 'nursery apartheid' by making parental leave more widely available."

'Protect government'

In her report, Mrs Blair said the original European document "does appear to have been mis-implemented".

The exclusion of parents of young children "cannot be justified " on grounds of flexibility, she wrote.

She said that while it was legally acceptable to breach the document by offering more benefits, there was a minimum requirement that needed to be met.

The case has stirred considerable interest, mainly because Mrs Blair is expecting the couple's fourth child.

But Mrs Blair is insistent she is not starting a rift within their Downing Street home, merely moving to protect her husband's government.

"Individual workers adversely affected could bring an action against the United Kingdom Government," she said.