BARONESS Castle of Blackburn today vowed 'I'll be back' after spending a night in hospital following a collapse in the House of Lords.

The former Cabinet minister sent a special message to her former constituents through the Lancashire Evening Telegraph.

She said: "Tell them I am OK. I'm mending and you can tell the people of Blackburn that I am recovering."

The 90-year-old was rushed to St Thomas' Hospital on Wednesday lunchtime, just across the River Thames from the Palace of Westminster after she collapsed in a toilet in the House of Lords.

She was kept in overnight for observation before being discharged yesterday lunchtime wshen she returned to her Buckinghamshire home.

An aide to the Baroness said the cause of her illness remained a mystery. After she collapsed she was attended by Lord Winston, one of Britain's leading obstetricians and fertility experts.

The faint happened shortly after Baroness Castle -- who served in Harold Wilson's Labour Cabinet -- spoke to the campaign launched by public sector union UNISON to close the pay gap between men and women in local government.

Union officials were impressed with her fighting spirit as were staff on the NHS ward at St Thomas' where she was soon "bright and alert" after what she described to colleagues as a "funny turn."