Under threat of legal action the Government is to extend employment rights to an extra 400,000 part-time workers.

Ministers had been warned by the Trades Union Congress that they would face litigation unless Britain's implementation of a European directive was strengthened.

Unions were furious over "boasts" by the Department of Trade and Industry (DTI) that only 45,000 out of Britain's 6.8 million part-time workers - the overwhelming majority of them women - would enjoy enhanced rights. Ministers have since changed their minds and now calculate that the new law will cover more than 420,000.

Part of the increase is because the DTI is prepared to substitute the word "workers" for "employees" in the regulations. That means the inclusion of casual staff, temporaries and homeworkers who do not have a contract of employment, but have a clear employment relationship with an organisation.

However, millions of part-timers who are unable to compare their terms and conditions with full-timers performing the same jobs are still excluded. It is understood that the DTI is not prepared to shift on the issue, insisting that "comparators" are necessary for workers to enjoy the new rights. It is unclear whether unions will withdraw threats of legal action if the Government stands its ground. Employees' representatives point out that equality legislation could be breached, arguing that women would be disproportionately affected by such a decision.

Ministers were due to implement the law last Friday, but were still said to be considering submissions from both sides of industry. Stephen Byers, Secretary of State for Trade and Industry, has let it be known that he will give industry eight weeks to prepare for the new regulations after they have been finalised and received parliamentary approval.

Employers have been highly critical of the way the Government has introduced laws such as the national minimum wage and parental leave. Guidance on the working-time regulations had to be rewritten twice because of errors and omissions.

A spokeswoman for the TUC said unions were not unduly concerned about the delay to the introduction of the laws on part-timers. "It is better to introduce effective rights late, than have inadequate ones rushed through," she said.

Under the new rights part-timers should receive the same hourly rate and the same pro rata entitlements to annual holidays and parental leave as comparable full-time colleagues.