MPs say policy differences between England and Wales are hindering progress in improving health, transport and higher education.
The Commons Welsh Affairs Committee said many points it had raised in its previous report on cross-border issues had been resolved but there were "serious" ongoing difficulties.
Policy differences must be tackled, it warned, urging the Assembly and UK governments to work together to address them.
There is a "serious and persistent" lack of data to compare how the NHS is performing in the nations, the committee said.
Committee chairman Dr Hywel Francis said: "We warmly welcome the progress which has been made since our original reports.
"Better communication between Wales and Whitehall is clearly taking place in a number of key areas where previously it was lacking.
"However, we continue to have serious concerns about particular issues where deficiencies remain.
"The relevant UK government departments must address our concerns and take action to ensure people crossing the Welsh-English border do not receive a lower level of service."
Welsh Assembly Government