Former Deputy Speaker Nigel Evans, who spent four days in Turkey as an official global monitor, said there was clear evidence of the campaign being one-sided.
"This is not some move we've done for the first time", said Guven, speaking to reporters Monday in Ankara.
Washington and Ankara have also disagreed over the U.S arming Kurdish militants in Syria, whom Turkey accuses of being "terrorists".
Few in Turkey expect legal challenges to the referendum to lead to a recount, let alone a re-run.
European electoral observers have criticised the referendum as not being held on an even playing field or in a genuinely democratic way.
The report also appeared to endorse the opposition's complaint that the country's electoral commission, which it accused of lacking transparency, changed its own rules on referendum day to order vote counters to consider valid ballots that lacked official stamps.
"Our regret is not over the outcome of the referendum, but because of the desire to overlook clear and harsh violations of the law that have the potential to impact the results", it said.
The People's Republican Party (CHP) said Deputy Chairman Bulent Tezcan would present the appeal to the YSK at 2:30 p.m.
The European Union is calling on the Turkish authorities to launch "transparent investigations" into "alleged irregularities" during last weekend's referendum on increased powers for the country's presidency.
On Tuesday, Yildirim said Erdogan would be invited to join the party as soon as the official results are declared.
"Such a speculative statement from a spokesperson can not be accepted", Turkey's EU Affairs Minister Omer Celik told reporters at a televised press conference, calling on the EU to "respect democratic processes". According to unofficial results, the margin of victory for the package of constitutional changes was just shy of 1.3 million ballots. This could enable President Erdogan to stay in power until 2029. Opponents accuse him of leading a drive toward one-man rule in Turkey, a North Atlantic Treaty Organisation member that borders Iran, Iraq and Syria and whose stability is of vital importance to the United States and the European Union.
ANKARA, Turkey (AP) - Turkey's prime minister on Tuesday called on the opposition to respect the result of a referendum that will give sweeping new powers to the office of the president.
"The people's will has been reflected at the ballot box, and the debate is over", Yildirim told his parliamentary group. "Everyone should respect the outcome, especially the main opposition".
Erdogan, meanwhile, called the referendum "the most democratic election. ever seen in any Western country" and admonished the OSCE monitors to "know your place". "Before our administration took over, there had been many decisions approving the validity of unstamped ballots". The CHP is collecting evidence before officially contesting the results with the election board and wants to eventually take the fraud allegations to Turkey's top court.