Workers are demanding compensation for travelling time

They claim it can take up to 90 minutes to reach Terminal 5 due to transport arrangements and heightened security.

About 500 Amicus union members working for 14 contracting firms are to be balloted and walkouts could start before the end of March.

Heathrow's operator BAA said the issues were "understood at the outset".

Amicus wants the workers reimbursed �25 a day for additional travelling time.

We reject the claims that it takes local workers an extra three hours to get onto site BAA

Amicus' Frank Westerman said: "Travelling time has been an issue on the site for the last two years but it has been exacerbated as security checks have been stepped up and as more workers come on site, causing even more congestion.

"Even our members who are living locally are facing an extra three hours on their working day just to reach and leave the site.

"Terminal 5 contractors are compensating other workers on the site for this additional time and we want the same for our members."

Security checks

A spokeswoman for BAA, which hopes the new terminal will be operational by 2008, said: "T5 poses unique logistics challenges due to its location as a secure construction site at the western end of the airport."

It added that these issues were recognised in negotiations that took place with Amicus for the Major Projects Agreement in December 2003.

She said: "We reject the claims that it takes local workers an extra three hours to get onto site."

BAA said it was "surprised and disappointed" by the decision by Amicus to consult their members over organising a ballot.

The sites steel erectors are being balloted for industrial action in a separate dispute after three workers were sacked.