Downing signed a new five-year contract with Boro last February, but Stewart Downing snr, his son's representative, said that promises made by the club at the time that the squad would be strengthened had not been honoured.
"Stewart has been frustrated since the summer," said Downing snr. "He was assured that there would be significant investment in the squad. He feels as though the club have not matched those expectations.
"He is also frustrated that the team is once again struggling in the Premier League.
"Obviously, as a Middlesbrough lad he has thought long and hard about the decision to hand in a transfer request and he also hopes that the fans will understand his reasons."
It is understood that Spurs chairman Daniel Levy has submitted an £11 million bid for Downing, who has 21 international caps, with fringe players at White Hart Lane thrown into the equation as well as England winger Aaron Lennon.
Boro said on Monday night that they had rejected the player's transfer request and it is understood that club officials are considering whether there are grounds to report Spurs to the Premier League for their conduct
Despite Spurs' advances, Boro remain determined to hang on to the star graduate of the club's acclaimed academy. Their chief executive, Keith Lamb, said: "Prior to receiving the transfer request, we received an improved offer for Stewart from Tottenham. We immediately rejected this offer and informed Tottenham that Stewart Downing is not for sale and any further interest they have in the player will not be entertained."
Middlesbrough do not want to sell Downing to one of their rivals near the foot of the Premier League, even though he has failed to score this season.
"It isn't a case of playing hardball, it is us saying Stewart is not for sale," said a club source. "Stewart will not be allowed to leave. Tottenham are nowhere near our evaluation but anyway what is the point of £15 million if the club is relegated."
While Boro defender David Wheater empathises with Downing, he still wants his fellow Teessider to stay put as Spurs step up their attempts to sign a player they have coveted for three years.
"The manager says he wants to keep our best players and that's what you have to do if you want to move up the league," Wheater, an England Under-21 international, said.
"I hope he wants to stay because being a local lad he loves the place and the club and hopefully he wants to do well.
"Do I see the day when Stewart might move on to a big club? You never know.
"That's what you want to do in your career – play for the best clubs and win trophies – so if we don't start doing it here maybe he might want to move on in the future.
"He will obviously want to be the best he can be at football. It's just like someone working in an office working their way up to be a manager. We just want to do the best we can.
"But hopefully he doesn't leave yet – we need everyone scoring goals at the moment."
As Downing's paymasters mulled over his request, the club's website gave an invaluable insight into their thinking following chairman Steve Gibson's assertion that Downing would not leave the club less than 12 months into a five-year contract.
"Steve Gibson insists that no senior players will leave the Riverside during January after the club turned down an offer from Tottenham for Stewart Downing," the website said.
"Following speculation in the media that Spurs were interested in Boro's England winger, the Boro chairman confirmed that a Spurs bid for Downing had been rejected.
"And both Gibson and manager Gareth Southgate reiterated previous statements that Boro were determined to hold on to the current first-team squad as the club looks to move up the Premier League."