The FTSE Group has released its quarterly review showing which companies have edged into the index and which have dropped out.
Tullow will now join the FTSE 250. Tullow has suffered a fall in its share price and recently reported a £1.3bn pre-tax loss - its first in 15 years.
Falling oil prices and concern over possible delays to West African projects have also hit Tullow's prospects.
The loss followed the company's revelation of a £1.5bn write-off at the beginning of the year. Inclusion in the FTSE rankings is dependent on market capitalisation.
Afren's share price nosedived today following the announcement it would default on £9.7m of interest payments that were due on 1 Feb. Afren has been dogged by fallen by oil prices and allegations of boardroom corruption.