The 31-year-old did not reach the Paralympic qualification times to be considered for selection at the British International Disability Swimming Championships in Sheffield over the Easter weekend, the last opportunity to qualify for London.
Roberts, an 11-time Paralympic gold medallist in the S7 category, missed the British Trials in March - the first opportunity to reach London qualification times - because of pneumonia and only returned to training three weeks ago after the green light from doctors.
However, the Welshman failed to reach the standard in the S7 50m and 100m freestyle at Ponds Forge at the weekend, much to his disappointment.
"You are selected on merit for this team, not the past," Roberts told C4Paralympics after his 100m race on Saturday. "If you were selected for winning 11 golds, I would get selected every time."
"We have a great mix of youth and experience - British Swimming head John Atkinson
However, defending Paralympic SB6 100m breaststroke champion Liz Johnson and European SB6 100m breaststroke gold medallist Charlotte Henshaw have been included in the ParalympicsGB squad, despite missing out on the 1.40.87 standard in either London or Sheffield.
The duo have been included subject to the number of places GB have been allocated by the International Paralympic Committee (IPC) for London 2012.
In total, 16 British swimmers, including double Paralympic gold medallist Ellie Simmonds, have been selected for ParalympicsGB, with a further 26 - including Johnson and Henshaw - to be ratified in early June.
"Those 26 athletes will therefore only have their selection confirmed by the British Paralympic Association (BPA) once the IPC slot allocation has been formally made in early June," said a BPA statement.
Beijing was really special but walking out in front of 17,000 people and hearing them cheer us on will be even better - Ellie Simmonds
Also included is veteran James Anderson, who will be competing in his sixth Games aged 49, while teenagers Gemma Almond (18), Jessica-Jane Applegate (18), Andrew Mullen (15) and Hannah Russell (14) will be among the 19 swimmers making their debuts.
"We have a great mix of youth and experience within this group and with 45% of the athletes preparing to take part in their first Games, I am sure our experienced athletes will help them settle into the routine of a Paralympic Games," said British Swimming's performance director for disability John Atkinson.
Double Paralympic champion Simmonds said the London Games are a "once in a lifetime opportunity" for her.
"Beijing was really special for me but I think walking out in front of 17,000 people and hearing them cheer us on will be even better," she said.
The team’s strength is reflected in the £10.4m of funding awarded by UK Sport for the 2009-2013 cycle, which was comfortably the highest amount given to any of the Paralympic sports.
Confirmed selected: Jessica Applegate (Great Yarmouth), Claire Cashmore (Leeds), James Crisp (Sheffield), Chloe Davies (Midsomer Norton, Somerset), Jonathan Fox (Cornwall), Heather Frederiksen (Leigh, Lancashire), Rhiannon Henry (Manchester), Oliver Hynd (Kirkby-in-Ashfield, Nottinghamshire), Sascha Kindred (Hereford), Aaron Moore (Trowbridge, Wiltshire), Andrew Mullen (Glasgow), Susannah Rodgers (London), Hannah Russell (Chertsey, Surrey), Eleanor Simmonds (Swansea), Matthew Walker (Stockport), Robert Welbourn (Swansea)
Nominations accepted, subjet to IPC Swimming confirming final athlete slot allocation: James Anderson (Broxburn, West Lothian), Gemma Almond (Shrewsbury), Jack Bridge (R) (Preston), Josef Craig (R) (Tyne and Wear), Graham Edmunds (R) (Swansea) Elizabeth Johnson (Bath), Sean Fraser (R) (Manchester), Charlotte Henshaw (Mansfield, Nottinghamshire), Emma Hollis (Epping, Essex), James Hollis (R) (Epping, Essex), Sam Hynd (Swansea), Natalie Jones (Chorley, Lancashire), Nyree Kindred (Hereford), Harriet Lee (Peterborough), Amy Marren (R) (Hornchurch, Essex), Natalie Massey (Stockport), Stephanie Millward (Swansea), James O’Shea (Newcastle), Daniel Pepper (Stockport), Benjamin Procter (Newquay), Craig Rodgie (Dunfermline), Lauren Steadman (R) (Huntingdon, Cambridgeshire), Anthony Stephens (Highbridge, Somerset), Louise Watkin (Leigh, Lancashire), Matthew Whorwood (Newquay, Cornwall), Thomas Young (Manchester)
Who attained Paralympic qualification standards for London 2012:
50m freestyle: Matthew Walker (S7)
100m freestyle: None
200m freestyle: Ben Procter, Dan Pepper, Craig Rodgie (all S14)
400m freestyle: Matt Whorwood (S6), Jonathan Fox (S7), Oliver Hynd, Sam Hynd, Thomas Young (all S8), Rob Welbourn (S10)
100m backstroke: Jonathan Fox (S7), Thomas Young (S8), James Crisp (S9), Aaron Moores, Craig Rodgie (S14)
100m breaststroke: James O'Shea (SB5), Sascha Kindred (SB7)
50m butterfly: Andrew Mullen (S5)
100m butterfly: None
200m individual medley: Sascha Kindred (SM6), Oliver Hynd, Sam Hynd (both SM8), James Crisp (SM9)
50m freestyle: Ellie Simmonds (S6), Susie Rodgers (S7), Louise Watkin (S9), Hannah Russell (S12)
100m freestyle: Ellie Simmonds, Natalie Jones (both S6), Susie Rodgers (S7), Heather Frederiksen (S8), Louise Watkin (S9), Hannah Russell (S12), Rhiannon Henry (S13)
200m freestyle: Jessica-Jane Applegate, Natalie Massey, Chloe Davies (all S14)
400m freestyle: Ellie Simmonds, Natalie Jones (both S6), Susie Rodgers (S7), Heather Frederiksen, Emma Hollis (both S8), Stephanie Millward (S9), Hannah Russell (S12)
100m backstroke: Nyree Kindred (S6), Susie Rodgers (S7), Heather Frederiksen (S8), Stephanie Millwward (S9), Hannah Russell (S12), Chloe Davies, Jessica-Jane Applegate, Natalie Massey (all S14)
100m breaststroke: Claire Cashmore (SB8), Harriet Lee (SB9)
50m butterfly: Susie Rodgers (S7)
100m butterfly: Stephanie Millward (S9), Hannah Russell (S9)
200m individual medley: Ellie Simmonds, Natalie Jones (both SM6), Louise Watkin, Stephanie Millward, Claire Cashmore, Amy Marren (all SM9), Gemma Almond (SM10), Rhiannon Henry (SM13)
- Find out more detailed explanations about classification on British Swimming's website