Thameslink links Bedford with Brighton by way of central London

Commuters were told to use the Northern Line Tube service to get through London after the rail service was suspended at 0925 GMT.

Dust was seen falling onto a southbound train as it passed beneath London's Kings Cross and St Pancras stations and engineers were called to make sure it was safe.

It was initially feared construction work on the Channel Tunnel Rail Link may have threatened the tunnel's foundations.

A Thameslink spokesman said: "Extensive construction work is being undertaken above ground in that area in connection with the Channel Tunnel Tail Link to St Pancras.

'No threat to tunnel'

"It appears now not to be as serious as far as we can determine.

"There was some concern that there may have been some damage to the tunnel roof."

It is thought the dust came from other building works which posed no threat to the tunnel, he added.

Southbound services terminated at Kentish Town and northbound services stopped at Blackfriars where commuters were told to use the Northern Line instead.

Landslip families evacuated

The rail service, which links Bedford with Brighton and Sutton, was expected to resume before the Friday rush hour.

Last month 50 people were evacuated from Stratford, east London after three gardens were swallowed up by a huge hole which opened up above the site of the Channel Tunnel Rail Link.

Work has since stopped on that tunnel and homes have been undergoing structural surveys to determine the extent of the damage.