West Ham are planning to lodge a complaint against Fulham after Roy Hodgson made six changes and lost 2-0 against Hull City on Saturday.
The win gave Hull a Premier League lifeline as they moved level on points with West Ham with a game in hand.
Gianfranco Zola's side only stayed out of the bottom three on goal difference after suffering a sixth consecutive defeat against Stoke City.
A West Ham source said: 'Roy Hodgson admitted on TV that he rested players because he had an important Europa League match coming up. He admitted fielding a weakened team. From our point of view that is awful and we are going to put in a complaint.'
The manner of Fulham's defeat fueled the bad blood that already exists between the London clubs.
Fulham are suing West Ham for around £500,000 - the loss of their league position bonus in 2006-07 after Carlos Tevez's goals helped keep the Hammers up and finish a place above Fulham in the Premier League.
West Ham were fined £5.5million in April 2007 by the Premier League for breaking rules over third-party agreements when they brought Tevez and midfielder Javier Mascherano to the club, but were not deducted points.
A West Ham source said: 'Fulham are suing West Ham for half a million quid. They may find West Ham suing them for £50m next season.'
Premier League regulations state that clubs must field full-strength sides in every match. But Hammers themselves rested Scott Parker and Carlton Cole in losing at Arsenal last month.
Wolves have already fallen foul of these rules this season and were given a £25,000 suspended fine after manager Mick McCarthy made 10 changes for a league match against Manchester United.
But Hodgson was unrepentant yesterday. The Fulham boss left out regulars such as Damien Duff, Danny Murphy, Dickson Etuhu, Aaron Hughes and Bobby Zamora, whose preparation had been hampered by a stomach bug, for the match at the KC Stadium in anticipation of the first leg of his side's Europa League quarter-final against VfL Wolfsburg.
He said "Firstly I don't thing West Ham should be picking our team and I think that is a fairly obvious statement."
"If there is a complaint my conscience is perfectly clear on the subject and we don't have a case to answer.
"If I had said we chose a team comprising a player just sold £10m and a player replaced with a South African international, and replacing Bobby Zamora with Clint Dempsey, a USA international, I can't understand why you can't changes things."
Hodgson spent the early part of Fulham's European campaign insisting Premier League survival was the club's priority. But now his attitude has clearly changed.
Sitting in mid-table, 11 points above the relegation zone, the Fulham boss can afford to prioritise a cup competition. 'We're entitled to focus more on the Europa League,' he said. 'Our performances haven't been worthy of any criticism, but there's no doubt that we've made a conscious decision that the cup competition will be more of a priority than the league because we're relatively safe.
'We looked at it and said we could still be OK, even if we lost every game in the league and the teams below us got more than they'd been getting. we can put the cup matches in the "shop window".'
But, for the teams for whom relegation is still a pressing threat, Fulham's Europa League run could have a real impact on the final league table.
Fulham play relegation strugglers Wigan on Sunday, and, should they beat Wolfsburg, a two-leg semi-final against Hamburg or Standard Liege would have to be squeezed in between matches against Wolves, Everton and West Ham, who play their final away game at Craven Cottage.
In Fulham's defence, tonight's match will be their 52nd of the season and their 15th in a European campaign that is entering its 10th month.