Lucas will leave his post on 16 August rather than the previously announced date of 28 February 2014.
Lucas said his health was a “key factor” behind his initial decision to step down, but continuing to work in his condition will “no longer be possible”.
I want to do the right thing by Barclays, my family, and myself, and therefore I have reached the difficult decision to step down sooner. I feel confident that I leave Barclays financially robust and well placed to continue to serve its customers, clients, shareholders and other stakeholders.
Chief executive Antony Jenkins said:
It is typical of Chris that last year when he told me that he wished to step down he offered to stay as long as he could while we found and inducted a successor. I am very grateful for his support, advice and wisdom during my first year as Chief Executive. Chris has helped navigate our company through many challenges during his six years with the business, and has played a key part in building the foundations which will ensure Barclays becomes the 'Go-To' bank in the years ahead.
Lucas will be replaced by JP Morgan's chief financial officer Tushar Morzaria.