Premiership Rugby, the umbrella organisation for England's top 12 clubs, announced on Wednesday a £152million, four-year deal with BT Vision.
The broadcaster has been sold rights for a three-year period from 2014 to screen live European games involving top English clubs, while their domestic competition arrangement starts next season.
That has effectively sounded a death knell for the blue riband Heineken Cup tournament, although the Rugby Football Union said last week it had not consented to Premiership Rugby granting European broadcasting rights.
European Rugby Cup - the organisers of the Heineken Cup and Amlin Challenge Cup - claim to do so is in breach of International Rugby Board regulations.
The deal comes less than a month after Premiership Rugby chief executive Mark McCafferty threatened an Anglo-French breakaway league - but Thomas insists the deal he signed on behalf of the RFU would prevent such a move.
"It stipulates the clubs must play in the two European competitions up to and including 2015-16," he told BBC Sport.
"If they go ahead with this breakaway they could be liable for a legal challenge and have to pay substantial damages."
McCafferty, though, said on Saturday: "The context everyone has to remember is that there is no European competition after 2014 for anyone because notice has been served on the accord so there is a vacuum in 2014.
"We are trying to decide - everyone is trying to decide - what should replace it. Whichever way, there will be new competitions after 2014."
A meeting is to take place in Dublin on Tuesday at which ERC stakeholders will negotiate over the television issue.