Emergency services are at the scene of a collision on Sand Lane, Marston, near Grantham.
The collision happened around 8.25am this morning and involves the cyclist and a car.
The male cyclist has been flown to the Queen’s Medical Centre in Nottingham with serious head injuries.
The road is likely to remain closed for some time.
Any witnesses to the collision, who have not already spoken with police, are asked to call 101, quoting incident number 80 of 3 July.