David "Dai" Rees Prosser (13 October 1912 – 6 May 1973 (aged 60)) born in Neath, was a Welsh dual-code international rugby union and professional rugby league footballer who played in the 1930s, and 1940s, and coached rugby league in the 1960s, playing representative level rugby union (RU) for Wales, and Glamorgan County RFC, and at club level for Glynneath RFC, Neath RFC, and Swansea RFC, as a Prop forward, i.e. number 1 or 3, and playing representative level rugby league (RL) for Great Britain, and Wales, and at club level for York, and Leeds, as a prop, i.e. number 8 or 10, during the era of contested scrums, and coaching club level rugby league for Leeds. Dai Prosser was a Bombardier in the British Army during World War II, he died in York.
Dai Prosser won 8 caps for Wales (RU) in 1936–1944 while at Glynneath RFC in 1934 against Scotland, and Ireland, won caps for Wales (RL) while at York, and Leeds, and won a cap for Great Britain (RL) while at Leeds in 1937 against Australia at Fartown Ground, Huddersfield.
Dai Prosser played right-prop, i.e. number 10, in Leeds' 2-8 defeat by Hunslet in the Championship Final during the 1937–38 season at Elland Road, Leeds on Saturday 30 April 1938.
Prosser played left-prop, i.e. number 8, in Leeds' 19-2 victory over Halifax in the 1941 Challenge Cup Final during the 1940–41 season at Odsal Stadium, Bradford, in the 15-10 victory over Halifax in the 1942 Challenge Cup Final during the 1941–42 season at Odsal Stadium, Bradford, he missed the 15-16 aggregate defeat by Dewsbury in the two-legged 1943 Challenge Cup Final during the 1942–43 season at Crown Flatt, Dewsbury, and Headingley Rugby Stadium, Leeds, and played left-prop in the 8-4 defeat by Bradford Northern in the 1947 Challenge Cup Final during the 1946–47 season at Wembley Stadium, London on Saturday 3 May 1947.
During Dai Prosser's time at Leeds, they appeared in four Challenge Cup Finals winning two (1941, and 1942), and losing two (1943, and 1947), he appeared in three of these Challenge Cup Finals, and was on the winning side twice.
Dai Prosser played right-prop, i.e. number 10, in York's 9-2 victory over Wakefield Trinity in the 1936 Yorkshire County Cup Final during the 1936–37 season at Headingley Rugby Stadium, Leeds on Saturday 17 October 1936.
Prosser's shared Testimonial match with David "Dai" Jenkins, Jr. at Leeds took place in 1949.
Dai Prosser played left-prop for Northern Command XIII against a Rugby League XIII at Thrum Hall, Halifax on Saturday 21 March 1942.
Dai Prosser was the assistant-coach to Joe Warham in Leeds' 25-10 victory over Warrington in the Championship Final during the 1960–61 season at Odsal Stadium, Bradford on Saturday 20 May 1961, in front of a crowd of 52,177.
Prosser was the younger brother of Wales, Glynneath RFC, and Neath RFC rugby union, and York rugby league footballer Glyn Prosser.