The Communication Workers Union (CWU) is taking an issue of pay discrepancy experienced by agency staff, such as those employed by BT, to Brussels this week.
Members of the union will be meeting with members of the European Parliament (MEP) to discuss the formal complaint submitted by the Trade Union Congress (TUC) last September, which charged the UK government with failing to implement the EU Temporary Agency Workers Directive (TAWD).
The CWU claims that the government's interpretation of the directive creates a loophole that allows companies, such as BT, to employ agency workers on a lower rate than a full-time staff.
For example, an agency worker at BT grade C2 is paid an hourly rate of £7.50, nearly £10 pounds less than a full-time member of staff on £16.98 per hour.
In addition, a BT permanent employee at grade B1CC is paid an hourly rate of £12.53, compared to an agency hourly pay rate of £6.31 to £7.80.
CWU said: "The UK government insisted on a derogation of the TAWD, whereby an agency worker is employed by an agency on a contract of employment and has a contract of a minimum of one hour. These contracts allow workers to be paid at least 50 percent of their basic pay or not less than minimum wage between assignments where the agency cannot find them work.
"These 'pay between assignment' contracts are facilitating companies such as BT and Manpower to employ agency workers on a much lower rate than a permanent member of staff."
However, BT said that it has "excellent" employee relations practices.
"In terms of agency resource, our procurement teams work closely with all our suppliers to ensure that all appropriate aspects of legislation, fairness and good practice are maintained in relation to the workers they supply to BT," a spokesperson for the company said.
"BT uses agencies for a number of reasons, including surges in demand. We have moved more than 3,000 agency staff to permanent roles within BT over the past five years. Not all people on agency contracts want to progress to a post with BT, however. Some find that the flexibility they can get with an agency contract is a better fit with their lifestyle or plans."