The proposal was touted as the UK's first high-occupancy vehicle (HOV) lane when it was announced almost three years ago.
At the time, the government stated that the project would be tested between junctions 7 and 10, Milton Keynes South to St Albans, and work would start in autumn 2005.
The scheme is part of plans to widen the motorway from three to four lanes, due to be completed next autumn. It was expected that the HOV lane would become operational from then.
But green transport campaigners are concerned that the project is under threat because there has been little information on progress since it was first announced.
Campaign for Better Transport roads and climate change spokeswoman Rebecca Lush urged ministers to commit to the HOV lane as they have done for a similar project on the M62.
Work began this week on a car-share lane on the motorway near Leeds. It is due to open in the spring, making it the first such scheme.
The 2.75km lane will allow vehicles with two or more people heading towards Leeds on the M62 to bypass traffic at the congested Chain Bar junction.
A Highways Agency spokeswoman denied that the M1 HOV lane is being dropped. "We are working out the detail. It is definitely in the plan," she said.